one of the first examples of that we saw earlier today the white house shut down and blocked testimony by a key player at the center of the ukraine scandal. >> reporter: tonight the white house declaring war with congressional democrats over impeachment. a letter from the trump administration to congressional leadership accusing them of a proceeding that violates the constitution. the rule of law and every past precedent. seeking to over turn the results of the 2016 election. the president lawyer writing your unprecedented actions left the president with no choice. president trump and his administration cannot participate in your partisan and unconstitutional inquiry under the circumstances. the letter coming at the end of a dramatic day that saw the white house block the testimony of one of the president's point men on ukraine. u.s. ambassador to the european union. house democrats firing back with