But House Speaker Paul Ryan broke sharply with the trend during on Thursday, telling CNN's Jake Tapper that he was not yet "ready" to endorse Trump.
"To be perfectly candid with you Jake, I’m just not ready to do that at this point. I’m not there right now," Ryan said. "I hope to though. And I want to. But I think what is required is that we unify this party."
Now, this doesn't appear to be a play to steal the nomination from Trump — because when pushed by Tapper, Ryan said again that he himself would not accept his party's presidential nomination.
Instead, it's a shot across Trump's bow — an attempt to warn the billionaire that he can't take Republican Party support for granted just yet.
Ryan repeatedly hammered home two things that, in his view, Trump would have to do to unify the party.
First, he kept saying Trump would have to be a conservative nominee. Ryan said, "I think conservatives want to know, does he share our values and our principles on limited government, the proper role of the executive, adherence to the Constitution?" Only when those questions are answered, Ryan said, could Trump "bring all wings of the Republican Party together."
This appears to be a warning to Trump not to swerve to the left on policy now that the general election is in sight. Trump has broken from conservative orthodoxy on trade policy, and his commitments to shrinking government and cutting taxes appear to many to be lip service. (Indeed, Trump had flip-flopped on his own tax plan's cuts for the wealthy earlier in the day, saying, "I am not necessarily a huge fan of that," and has never been a big fan of Republican plans to cut Social Security and Medicare spending.)
Second, Ryan stressed that Trump needed to have a message that would "appeal to all Americans in every walk of life, every background, a majority of independents and discerning Democrats."
Here, Ryan appears to be urging Trump to tone down the anti-immigrant demagoguery specifically, and the offensive statements more generally. (When Republican leaders talk about Americans of different "backgrounds," they usually mean, "Hey, if we don't do better with Hispanics, we'll lose.")
In effect, Paul Ryan is urging Trump to run the campaign that Jeb Bush or Marco Rubio would have run — to be an orthodox conservative tax-cutter and spending-cutter, while trying to win over enough Hispanic voters to actually get to the White House.
Yet Trump has been running essentially the opposite campaign and has had enormous success. And it's not clear what leverage Ryan has over him at this point, with so many other party figures unifying behind Trump, and the convention coup possibility apparently off the table.
So the ball is in Trump's court now, and we'll see if he's interested in making a deal — or if he'll tell the speaker of the House to buzz off.
Update: It's official: Trump's choice is to tell the speaker of the House to buzz off: