Donald J. Trump is stamping out the final flashes of independence inside Republican institutions with astonishing speed, demonstrating that his power continues to expand over the new party establishment he has created.

At the Republican National Committee, he is moving to replace longtime supporters with allies even more closely bound to him, including his daughter-in-law, Lara Trump.

In the House, Republicans are more compliant than ever. Most vividly, Speaker Mike Johnson — ostensibly the party’s top-ranking official — backtracked on an endorsement in a crucial Senate race because Mr. Trump disagreed. On Thursday, Mr. Johnson’s candidate ended his campaign less than one week after opening it.

In the Senate, which has been less beholden to Mr. Trump, his influence over a failed border bill made one of the party’s most effective lawmakers, Mitch McConnell, look weak.

The displays of obedience emerging in recent weeks remove any lingering doubt that the Republican Party is aligned to advance the interests of one man, signaling that a sweep of victories from Mr. Trump and his allies in November could also mean replacing checks and balances in Washington with his wishes and whims.

For many Republicans, those aren’t risks but the rewards of a second Trump administration. Only a rapidly dwindling minority inside the party remains worried about Mr. Trump’s intentions.