Last week, President Trump celebrated his grip over the Republican Party.

“Look at what happened last night,” Mr. Trump boasted at a cabinet meeting on Wednesday, after yet another MAGA-aligned candidate he had endorsed walloped an establishment Republican, this time in the Texas primary. “That was the prelude to the midterms.”

But it’s far from clear that Mr. Trump’s winning streak in the Republican primaries will translate into victory in November, when the party will need the support of voters outside of Mr. Trump’s base — many of whom are deeply dissatisfied with the economy and the Iran war.

Already, there are signs that his hold over Republicans in Washington may be slipping.

In a rare show of dissent, Senate Republicans earlier this month blocked Mr. Trump on money for his White House ballroom and pushed back on a $1.8 billion fund to reward Trump supporters who claim political persecution by Democrats. The opposition was so intense that the White House has considered scrapping it.

Mr. Trump has also been blocked by the courts: a federal judge on Friday temporarily barred Mr. Trump from setting up the fund for his supporters, and another court ordered him to remove his name from the John F. Kennedy Center for the Performing Arts. The Kennedy Center ruling prompted an incensed president to consider backing down — something he never likes to be seen doing.

“We are going to be working with Congress to transfer this failing Institution back to them so they can make a determination as to what to do with it,” Mr. Trump said as he complained that there had “never been a President of the United States who has been treated so unfairly by the Courts as I.”

In a matter of days, Mr. Trump had gone from dominant in the primary races to facing enormous pushback by Republican elected officials in Washington and the courts.