The White House on Monday blocked former counsel Donald McGahn from testifying at a congressional hearing scheduled for Tuesday, the latest act of defiance in a deepening fight between Democratic lawmakers and President Donald Trump.

A White House attorney said in a letter that McGahn should not appear due to "constitutional immunity" and "to protect the prerogatives of the Office of the Presidency." Democrats issued a subpoena last month for McGahn to testify, and hoped that he'd be a star witness in the House Judiciary Committee's investigation into whether Donald Trump obstructed justice.

The Democratic chair of the House Judiciary Committee said last month that "McGahn is a critical witness," and said he would hold the attorney in contempt if he didn't show up.

"We're fighting all the subpoenas." Trump declared last month that his administration would refuse to cooperate with every congressional probe, and has dismissed them as political harassment to score points ahead of the 2020 elections.

In most of the cases where Trump and his advisers are refusing to cooperate, they run the legal risk of contempt of Congress citations and court enforcement actions that could result in fines and even imprisonment.