In the last four presidential elections, the Republican nominee has never won less than 90 percent of the Republican vote. Republicans won the popular vote only one of those times, in 2004, and when they did they carried 93 percent of Republican voters. Donald Trump has defied the odds before, but they are against his achieving this degree of party unity.

Trump has won more primary votes than any previous nominee, and he is going to add to that number in the weeks to come. But he will also be a nominee with a record number of primary votes cast for his opponents.

And to say the race has been divisive is an understatement: The day he clinched the nomination, he suggested, insanely, that Ted Cruz’s father was somehow involved in the assassination of John F. Kennedy. Cruz responded by mentioning that Trump once bragged about his close calls with venereal disease.

A lot of Republican officeholders — past, present and would-be-in-the-future — will rapidly endorse Trump, regardless of what they previously said about his psychological stability, honesty and all-around fitness for office. Trump’s numbers in polls against Hillary Clinton may improve as many Republican voters accept him as their nominee. (Then again, they may not, depending on whether Democrats quickly start running ads attacking him.)

Some Republican officeholders and voters will still consider Trump unfit for office and no conservative — but his supporters, old and new, will try to shame them into supporting the presumptive nominee. Newt Gingrich, who has for months been backing Trump without explicitly endorsing him, told radio host Sean Hannity on Tuesday night that if you’re not for him you’re “functionally for Hillary Clinton” and therefore for a “radical Supreme Court.”

Gingrich and other Trump supporters will have to overcome four problems in making this case.

First, Trump himself has said quite recently that he does not need Republican unity to win the election. So Gingrich is asking Republican voters who have serious misgivings about Trump to support him even though he himself is hardly asking them to do so.

Second, his most vocal supporters have spent the primaries trashing the Republican Party and declaring anti-Trump conservatives irrelevant. It’s a little late to flip the script now. Third, the message — vote Trump, you idiot crybabies — may just fall flat because a lot of voters dislike being addressed this way.

Fourth, conservatives don’t have much reason to trust Trump on the Supreme Court. He has given no indication of caring about judicial conservatism. If he put up non-conservative nominees, he would win praise for his bipartisanship and could make one of those deals with the Democrats he is always promising. What would hold him back? His word?

Fifth, and I think most important, Trump may spend much of the campaign too far behind Clinton for all this shaming to have any force. If the candidates are neck and neck, then maybe Republicans on the fence could be swayed by the argument that they have to vote for Trump or see Clinton in power. If Trump is behind by eight points, on the other hand, then a lot of Republican voters won’t see themselves as having to make the agonizing choice to back Trump to stop Clinton.

If Clinton has a consistently large lead, Trump won’t get the 90-plus percent of the Republican vote that his predecessor nominees got. A significant fraction of Republican voters will feel free not to vote for Trump because they will assume, accurately, that Clinton will be president whatever they do. And the root cause of that assumption will be that Trump is such a weak general-election candidate.

In that case, the functional allies of Clinton, the enablers of her judicial nominees, will turn out to have been those, like Gingrich and Hannity, who helped that weak candidate win the Republican nomination.

(c) 2016, Bloomberg View · Ramesh Ponnuru