Donald Trump in Omaha on Friday. (Charlie Neibergall/Associated Press)

Donald Trump lashed out at House Speaker Paul Ryan (R-Wis.) for saying he is not ready to support him, but it is not Ryan that Trump needs to worry about. Rather, it’s the majority of Republican voters who did not vote for Trump — and may not do so come November.

Trump won just 40.2 percent of all votes cast in the GOP primaries, the lowest percentage of any Republican nominee since Richard Nixon in 1968. Except for Sen. John McCain (Ariz.), no Republican presidential nominee in nearly five decades has failed to win a clear majority of GOP primary votes.

Trump brags that he is on track to break the GOP record for the largest number of total primary votes of any GOP presidential nominee in modern history — about 10.7 million so far. But he has also set another record: the most Republican primary votes against his candidacy — 15.9 million — of any GOP presidential nominee in modern history. Almost 60 percent of GOP voters wanted someone else to be their nominee.

Right now, Trump faces an uphill climb to win over those voters. A recent USA Today/Suffolk University poll shows that 4 in 10 Republicans aren’t sure whether they will vote for Trump in November. Among the majority of GOP voters who cast ballots for other candidates, 25 percent say they will seriously consider a third-party candidate and 19 percent may vote for Hillary Clinton, while 18 percent may stay home.

Trump also has a big problem with Republican women. An NBC News/Wall Street Journal poll in March found 47 percent of Republican women primary voters said they couldn’t imagine voting for Trump in November.

If Trump does not change the minds of these Republicans, he is going to lose to Hillary Clinton.

Trump said this weekend he does not believe the Republican Party needs to unify behind him in order for him to ascend to the White House. He’s wrong. Right now, Trump has just 5 percent support among Democrats — that’s less than Mitt Romney, who won 7 percent of Democrats and 93 percent of Republicans in 2012 and still failed to win the presidency. Trump’s protectionist trade message and his attacks on China and Mexico may boost his numbers among Democrats somewhat, but there just aren’t enough disaffected white male Democrats to make up the difference if he does not consolidate his conservative base. Without Republicans united behind his candidacy, he has no chance.

Ryan spoke for millions of Republicans when he said that, while he wants to support Trump, he’s “not there right now” because he is unsure where Trump stands on “our principles of limited government, the proper role of the executive, adherence to the Constitution.” There is good reason to be concerned. Only days after locking up the nomination, Trump is already backtracking on conservative positions he staked out during the primaries.

For example, Trump won a lot of praise from conservatives in September when he rolled out his tax plan. Stephen Moore, chief economist at the Heritage Foundation, called it “Reaganesque,” adding “Who would have known Donald Trump was a supply-sider?” Now, Trump is backing away from his plan, declaring that he’s “not necessarily a huge fan” of all of its elements and that it is merely a starting point for negotiations because “Democrats and everybody else, they’re going to come to me. They’re going to want to negotiate.”

So the master of the “art of the deal” is making concessions before negotiations even begin? Is that what they teach at Trump University?

Trump also flip-flopped on the minimum wage. During the fall campaign, Trump said, “We have to leave [the minimum wage] the way it is” because “we have to become competitive with the world.” But when asked about a minimum wage increase this week, he said “I would like to see an increase of some magnitude.” Asked whether he was changing positions, Trump scoffed: “Sure, it’s a change. I’m allowed to change. You need flexibility.”

Conservatives look at this and ask themselves: What’s next? Abortion? His promise to increase defense spending? Naming a conservative to replace Justice Antonin Scalia on the Supreme Court? Will he stick by any of his campaign pledges, or is every conservative position Trump has taken negotiable?

House Speaker Paul Ryan says he cannot support Donald Trump. Trump says he doesn't care. Can the two work out their differences? (Deirdra O'Regan/The Washington Post)

Trump said this weekend, “I’m a conservative, but don’t forget, this is called the Republican Party. It’s not called the Conservative Party.” That should set off alarm bells for the party’s conservative majority.

Trump may not care whether Ryan endorses him, but he does need to worry about the millions of Republicans who share the speaker’s concerns about Trump’s conservative bona fides. There are a lot of conservatives who, like Ryan, are “not there yet” when it comes to backing Trump. And if Trump does not get them there, he won’t get to the White House — and then he won’t have to worry about offering concessions to Democrats next year.

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