“I think he’s completely winning the expectation game,” said Representative Peter Roskam of Illinois. “I think he’s a genius at lowering expectations and overperforming,” he said, adding, “It’s really remarkable.”

In one significant way, congressional Republicans potentially seemed to pull Mr. Trump to their end of the policy pool. On Thursday, the administration initially appeared to endorse taxing imports as a way to pay for the Mexican border wall, reversing its earlier preference for imposing a heavy tax on companies that move jobs overseas. But the White House later said it was just one option under consideration.

“We are in a very good place on tax reform,” Mr. Ryan said. “It can get complicated when you get into the details of tax reform, but once we go through how tax reform works and what it’s going to take to get the kind of competitive tax system, the kind of competitive tax rates, I think most people agree that this is the right approach.”

Congressional Republicans are also struggling to keep up with Mr. Trump’s rapid-fire announcements, let alone push their agenda. “It’s fast-paced stuff,” said Senator John Hoeven, Republican of North Dakota. Investigating voter fraud, for instance, is not something he would like to see Congress take on. “Our priorities are the ones we laid out,” he said.

They are also eager to get on with the rest of that agenda — specifically a repeal and, ostensibly, a replacement of the Affordable Care Act. “We are on the same page with the White House,” Mr. Ryan insisted Thursday. “The president agrees with this agenda.”

But it is the sudden embrace of federal spending that represents perhaps the most striking departure, with Republicans backing the concept of starting the financing for the border wall with a new appropriation.

And the list is much longer. By contrast, last year, Senator John Cornyn of Texas, the No. 2 Senate Republican, called Democrats’ request for $600 million in aid to Flint added to an energy bill “a huge earmark,” adding, “I think it’s not something I could support,” in keeping with most of his colleagues. Republicans also pushed for and partly succeeded in offsetting a bill to fight Zika last year.