WASHINGTON — Even after President Trump sued him last month to keep his business records secret, Representative Elijah E. Cummings kept his cool and urged Congress to move slowly on impeachment. But with Mr. Trump manning a full-scale blockade of Democrats’ access to documents and witnesses, the ordinarily careful Democrat is, like the rest of his caucus, growing impatient.

“It sounds like he’s asking us to impeach him,” Mr. Cummings, the chairman of the House Committee on Oversight and Reform and a top lieutenant to Speaker Nancy Pelosi, said in an interview last week. Ticking off all the ways Mr. Trump is stonewalling Congress, he added, “He puts us in a position where we at least have to look at it.”

Mr. Cummings’s remarks, which have been echoed by Ms. Pelosi, represent a significant shift for top Democrats, who have been trying to maneuver carefully around the impeachment issue. But with Mr. Trump standing in the way of their investigations — on Wednesday he asserted executive privilege over the unredacted version of the special counsel’s report and on Tuesday he tried to block the former White House counsel from handing over documents — their strategy of holding impeachment-like hearings without declaring a formal impeachment process is looking like a dead end.

The frustration is showing.

Mr. Cummings called the White House effort to block multiple lines of inquiry “far worse than Watergate.” He sees a “constitutional crisis” that even the founding fathers did not envision when they created the system of checks and balances that has kept American democracy intact.