push to ban it. "nightly news" begins right now. good evening, there are exactly three weeks to go until election day. and with donald trump continuing today to sew seeds of doubt about the integrity of the voting system, president obama jumped into the middle of things, advising trump to stop whining. trump's conspiracy theories and mantra about a rigged election have come to define much of the home stretch of the campaign, along hillary clinton's daily e-mail embarrassments. if there's a shake-up in the state of this bitter race coming, it could come tomorrow night when the two meet in las vegas for their final debate. that's where hallie jackson leads us off tonight. hallie? >> reporter: hi, lester. donald trump's aides say he spent part of his morning preparing for the final face-off. he's expected to bring president obama's half brother as one of his guests here tomorrow, a signal everything is now on the table. donald trump tonight complaining of corruption -- >> voter fraud is all too common. >> reporter: to president obama, he's just complaining. >> i'd advise mr. trump to stop whining and go try to make his case to get votes. >> reporter: the president slamming trump for his rhetoric about a rigged system. >> i have never seen in my lifetime or in modern political history any presidential candidate trying to discredit the elections. >> reporter: even some republicans like marco rubio agree trump should stop stoking voter fraud fear in the face of overwhelming evidence our elections aren't rigged. >> they even want to try to rig the elections at the polling booths where so many cities are corrupt, and you see that. >> reporter: trump, defiant and undaunted, two words that describe his final three weeks, trailing hillary clinton, he's now trying to shake up the race by showing he can shake up washington. proposing term limits for members of congress. he's running against the establishment, but still chased by his own controversies, like an accusation of sexual misconduct against him from a former "people" magazine writer. not verified by nbc news, and denied by trump, the magazine today publishing the accounts of five people who say the writer told them what happened at the time, and another who said she was with the writer when they encountered melania trump later in new york. >> it was not true. so how we could believe her? that never happened. to turn the tide, trump needs to rally his base to make sure they show up in three weeks. >> every time there's been open running room for donald trump, he doesn't advance the ball. tomorrow night's his last chance. >> reporter: in the audience as his guest tomorrow, the mother of shaun smith, who blames hillary clinton for her son's death in the benghazi attack. for trump, a focus on clinton's controversies instead of his own. hallie jackson, nbc news, las vegas. >> reporter: i'm andrea mitchell. tonight hillary clinton arriving in las vegas as the clinton team prepares for the home stretch. the strategy, in the debate, stick to issues, try to appear presidential, no matter what donald trump does. with trump trying to ralth clinton by bringing the mother of an american killed in benghazi to the debate. >> benghazi, there have been seven to ate congressional investigations on. >> that was all for the people who died, jen. >> we think it's a resolved issue in the voters' minds. >> reporter: also dealing with the torrent of stolen e-mails released by wikileaks. tonight, new messages, one from campaign chairman john podesta calls bernie sanders a dufus. another, three months before clinton nailed the nomination, podesta listing potential running mates in what he calls food groups, dividing them by race and gender. on the list, apple's tim cook, bill and melinda gates, sanders down at the bottom of the e-mail in a category all by himself. despite those e-mails, tonight team clinton sending sanders to arizona, going for a win in red states. michelle obama going there thursday. >> mr. trump is the least qualified, the least qualified candidate in the history of this country. >> reporter: and clinton getting more backup tonight, after an fbi agent said he felt pressured by the state department not to label one of clinton's e-mails classified, a charge tonight by both the fbi and the state department. president obama weighing in. >> i think you've heard directly from both the fbi and the state department that the accounts that have been put out there are just not true. >> reporter: clinton aides tonight just laughing off donald trump's recent charge that she is doping for the debate. her schedule after tomorrow night's debate, nonstop travel to the battleground states, ohio, pennsylvania, north carolina, new hampshire, and florida, all in the next week. lester? >> andrea mitchell tonight, thanks. and for more now on where the election stands and what we should be looking for heading into tomorrow's final dwad, i'm joined by our political director and moderator of "meet the press," chuck todd. when you look at the polls over the many months, the margins have changed here and there, but opinions of these two candidates haven't changed that much. we're into the final three weeks, the big debate tomorrow, what could happen that might change things? >> to me, that's the most important question. here we've had ten straight months of polling and campaigning by these two, and the negative feelings at the start of this year that the public had for both of them are still there. they feel more negative about him than her. so the question is, is there anything in the next three weeks? obviously, if you're trump, you need to make this debate work big time. i think the only thing that can change trajectory in this race is some unknown incident, because at this point, i think everything feels like it's starting to settle into concrete here. the only time we've seen anything move at all is when we heard the words of donald trump in that videotape. that's the type of issue that would have to pop up to actually change the direction of this campaign at this point. >> who has more riding on tomorrow? >> oh, it's donald trump by a mile. he has to have a great debate, almost a knockout performance in order to change the direction of this race. >> all right, chuck todd, thank you very much. we'll have full coverage and analysis of tomorrow's final presidential debate in las vegas, starting at 9:00 eastern, 6:00 pacific right here on nbc. turning now to iraq and the american-supported campaign to take back the city of mosul from isis. president obama said today what troops on the front lines have already discovered, it's going to be a difficult fight. so far commanders say the operation is going according to plan, but are warning about what isis may do overseas in retaliation. our chief foreign correspondent richard engel is in the region and spoke exclusively with the general leading the american effort. >> reporter: how's it going? >> it's going well. >> reporter: in his first interview since the offensive to push isis out of mosul began, major general gary valesquey said his troops are providing air support and heavy artillery, but not leading this charge. >> the iraqis have the momentum, they know it, and they want to get there as quickly as they can. >> reporter: we followed iraqi troops to the front line to see for ourselves. we made our way to the newly liberated village of hud. she's happy said a man ush approximating his 80 grandfather. she's happy to see the soldiers. but why were these people carrying white flags? because iraqi troops have moved on, even though villagers say there are still isis fighters here. and what we saw next was a war planner's worst nightmare, entering this sunni village were shi'ite militias. in a country that's seen civil war between these two sects of islam, it's a dangerous mix. but what happens here could have international consequences as well. general valesquey is concerned that in retaliation for losing mosul, isis could launch terror attacks on targets in the west. >> that's what we're worried about from my perspective, back at home, when mosul falls, how are they going to try to deflect the attention? >> reporter: but for now the troops are focused on the task at hand. mosul is where the so-called islamic state or caliphate was born. >> there's not going to be a caliphate. >> reporter: this battle is just beginning, and these are just the first of what aid agencies could be more than a million people driven from their homes. they suffered under isis, now they're paying the price for yet another fight for mosul. while u.s.-backed iraqi troops are advancing toward mosul, isis is saying that nothing is wrong. its propaganda today putting out images of people at work, eating cab ab in the city, life as normal. lester? >> richard engel, thank you. back home, much of the eastern half of the country remains in the middle of a fall heat wave this evening, while new york was one degree short of a record new high for this date. highs for this date were set throughout the northeast and down to texas with temperatures in the mid to upper 80s. more of the same is predicted for tomorrow when dozens of records could be shattered yet again. indian summer in full swing. now to the brutal and shocking execution caught on camera in mexico. a federal judge who reportedly played a role in joaquin "el chapo's" guzman's extradition process to the u.s. was shot in the back of the head in broad daylight while taking a jog. >> reporter: the shocking surveillance video shows the judge jogging when a gunman runs up from behind, pulls a handgun, then the trigger. a point blank execution of the judge, a 37-year-old mexican judge was reportedly involved in the extradition process of drug king pin joaquin "el chapo" guzman, and other high profile cartel members. addressing the nation, mexico's president promised a full investigation. el chapo is behind bars in federal prison, but experts say he still has connections to order brutal hits. last month, mexico's army says el chapo's sons were behind an ambush that left five soldiers dead. el chapo's lawyer says the widely feared drug lord who escaped from prison twice using tunnels like this one, was not involved in the murder. the judge also presided over other cartel cases and had not requested personal security. >> mexican drug trafficking organizations have a strategy to corrupt high level government officials, take my silver or take my lead, and it's been very effective. >> reporter: with el chapo facing extradition to the u.s. next year, the investigation into this brutal hit is just beginning. a crime designed to send fear through a nation doing exactly that tonight. miguel almaguer, nbc news. there is news tonight for millions who depend on social security. don't expect a big raise coming next year. retirees will get a .3% increase in monthly benefits, that adds up to about $4 a month for the average recipient. the average monthly social security payment now $1,238. the cost of living adjustment will be small because inflation is low and that's due in part to low gasoline prices. still ahead tonight, a warning for parents, an alarming rise in overdoses and deaths from a new drug more powerful than heroin being purchased by teenagers legally online. also, he has a way with words, this is why the nobel committee awarded him the literatu (burke) at farmers, we've seen almost everything, so we know how to cover almost anything. even mer-mutts. 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