But Mr. Trump’s inner circle has shrunk, and he has fewer advisers around him whom he trusts. His White House chief of staff is still serving in an acting capacity, and the West Wing is depleted by the shutdown. As he himself wrote on Twitter this weekend, “There’s almost nobody in the W.H. but me.”

Mr. Surabian said the rest of the party must recognize the threat and rally behind the president. “Republicans need to understand that Democrats in Congress, beholden to the ‘resistance,’ aren’t interested in bipartisanship, they’re out for blood,” he said. “It’s a war we can win,” he added, “but only with fortitude, unity, coherent messaging and a willingness to fight back.”

Democrats, for their part, say they are out for accountability, not blood, intent on forcing a president who went largely unchecked by a Republican Congress during his first two years in office to come clean on the many scandals that have erupted involving his business, taxes, campaign and administration.

They plan to get started in the coming days. On Tuesday, they will grill former Attorney General William P. Barr, who has been nominated by Mr. Trump to assume his old office again, about his approach to the special counsel, Robert S. Mueller III. Mr. Barr wrote a private memo last year criticizing Mr. Mueller’s investigation, and Democrats will use his confirmation hearings to press him on whether the special counsel will be allowed to finish his work and report it to Congress.

Senator Chuck Schumer of New York, the Democratic minority leader, also plans to force a vote in the Senate this week on the Trump administration’s plans to lifts sanctions on the companies of Oleg V. Deripaska, a Russian oligarch close to Mr. Putin’s government, if he reduces his ownership stakes. Democrats plan to use the issue to argue that Mr. Trump has been soft on Russia.

Even committees that are not usually in the investigation business are jumping into the fray. Representative Eliot L. Engel, Democrat of New York and the new chairman of the House Foreign Affairs Committee, told The New Yorker last week that he was eliminating the subcommittee on terrorism in favor of a subcommittee aimed at investigating Mr. Trump’s foreign policy.

Lost in all this may be any chance of bipartisan policymaking. At stake in the current fight is just $5.7 billion for Mr. Trump’s promised border wall, roughly one-eighth of one percent of the total federal budget. If one-eighth of one percent of the total budget can prompt the longest government shutdown in American history, then the potential for further clashes over the remaining 99.87 percent seems considerable. On issues like health care, taxes, climate change, guns and national security, the two sides start this era of divided government far apart.