“They stick together,” Mr. Trump added. “You never see them break off.”

It was the second time in as many days that he has complained about a lack of support from Republicans.

“When do the Do Nothing Democrats pay a price for what they are doing to our Country, & when do the Republicans finally fight back?” Mr. Trump tweeted late Sunday night.

The president’s allies on Capitol Hill tried Monday to ramp up their defense of the president by forcing a vote in the House to censure Representative Adam B. Schiff, the California Democrat who is leading the impeachment inquiry as the chairman of the Intelligence Committee. The vote, which failed in the Democratic-led chamber, was a display of Republican solidarity for Mr. Trump.

There are risks for Democrats in the longer timeline, which could make it more difficult for lawmakers in politically competitive districts, who fear a backlash from constituents if they appear to be preoccupied with targeting Mr. Trump instead of addressing major issues such as gun safety or health care.

And Democrats are all too aware that Mr. Trump has succeeded in the past in steering the subject away from allegations of misconduct on his part, as he did with the investigation into Russia’s interference in the 2016 presidential election conducted by the special counsel, Robert S. Mueller III.

This time, Democratic leaders hope to deny him the opportunity.

They have issued subpoenas to a growing cast of characters, including Rudolph W. Giuliani, Mr. Trump’s private lawyer who is at the center of the Ukraine pressure campaign, and have demanded documents from Vice President Mike Pence. They have invited or compelled Trump administration officials past and present to appear at the Capitol before rolling television cameras, and cloistered them behind closed doors to extract a daily drip of testimony that backs up their case.

That effort continues Tuesday when William B. Taylor Jr., the top American diplomat in Ukraine, is scheduled to testify behind closed doors about text messages in which he wrote to other officials that it was “crazy to withhold security assistance for help with a political campaign.” On Wednesday, investigators will question Laura Cooper, a Pentagon official, about decisions to hold up Ukraine’s military aid.