Transcript for President Trump claims 'we're fighting all the subpoenas'
Now to president trump and what he's saying about impeachment and the supreme court. As he also says he'll block subpoenas from congress. Here's Jonathan Karl. Reporter: Setting up a major showdown with congress, president trump declared today he won't cooperate with their investigations. Well, we're fighting all the subpoenas. Look, these aren't, like, impartial people. Reporter: House Democrats have launched several investigations into the president and his administration. Already issuing a subpoena to force former white house counsel don mcgahn to testify. Mcgahn is a ntral figure in the Mueller report, cited more than 150 times. The president does not want him talking to congress. Well, the subpoena is ridiculous. We have been -- I have been the most transparent president and administration in the history of our country, by far. Reporter: And then there is the fight over the president's tax returns. The treasury department has already missed two deadlines to turn them over. The president now argues that since the Mueller report is out, there's really nothing left to investigate. I thought after two years, we would be finished with it. No, now the house goes and starts subpoenaing. They want to know every deal I've ever done. Reporter: Democrats are outraged. We now see the administration engaged in stonewalling of the facts coming to the American people. Reporter: The president insists it's all politics. The Democrats are trying to win 2020. They aren't going to win with the people that I see and they're not going to win against me. Reporter: And he offered a novel response today to democratic calls for impeachment, tweeting, "If the partisan DEMs ever tried to impeach, I would first head to the U.S. Supreme court." The supreme court, however, has no power to overturn an impeachment. Jon, we saw there in your piece how Nancy Pelosi, asked about impeachment. She said she wants hearings to get the facts first. But the president is trying to block the subpoenas. What kind of power does congress have? Reporter: Congress does have the power to issue subpoenas, but what happens if the white house refuses to comply? This may have to be resolved in the courts, and almost certainly, the supreme court. Jon, thanks.
This transcript has been automatically generated and may not be 100% accurate.