Donald Trump speaks at the California Republican Party convention in Burlingame, Calif., in April. (AP Photo/Jeff Chiu)

Now that Donald Trump has all but wrapped up the Republican Party’s presidential nomination, the party’s elites are divided over whether or not they will support him. Some, including 1996 presidential nominee Robert J. Dole and former vice president Richard B. Cheney, have endorsed Trump. Others have not, at least for now, most notably including House Speaker Paul D. Ryan, arguably the most powerful leader in the GOP-controlled Congress. Others even say they will not vote for him, including former candidate Jeb Bush, former presidents George H.W. Bush and George W. Bush, and 2012 presidential nominee Mitt Romney. Republican Sen. Ben Sasse of Nebraska went so far as to call for a third-party candidate.

Is it the duty of a loyal party member to support the party’s presidential nominee, regardless of what one thinks of that person’s personality, policy positions or suitability? Should “good” Republicans support Trump? As it happens, there’s a great deal of research that speaks to this question.

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)

Political science has thoroughly examined how politicians benefit from joining a political party

Political scientists studying U.S. politics have shown what benefits politicians get from party membership. John Aldrich best articulated the argument that political actors turn to parties in order to  “win” more often — not just elections but the various battles that come before and after. That is, parties help politicians coordinate with others who generally share their political points of view, enabling them to pass more legislation and win more votes.

In congressional politics, parties serve as a “long coalition” in which members agree to vote together – even on issues where they disagree – so that, over time, they enact more policies than they could have alone. Voting together consistently also helps create a clear party brand, which citizens can use as a shortcut in deciding how to vote.

Political scientists thus typically see party membership as a positive for politicians.

But political scientists don’t often discuss the costs of joining a party

For parties to function, politicians agree to give up some of their individual liberty in exchange for the benefits the party provides. That is, for the party to be successful, party members will on occasion have to vote against what they or their constituents prefer.

In Congress, majority party leaders work to prevent votes on legislation that most of their members oppose. This partisan gate-keeping sometimes hurts the interests of some individual party members. Likewise, majority party leaders pursue legislation that most of the party prefers, and expect party members who disagree to support those bills anyway – especially if their votes are needed to pass the bill into law.

Party members generally accept this. They know that to capture a party’s benefits, they must also accept the costs. For example, when they are on the losing side of a debate within the party, they accept that they will not get what they want and fall in line for the greater good. That’s the price of being on a team.

All that is true for presidential nominations, too. Usually.

A similar logic applies to presidential nominations. Capturing the nomination of either party is not easy. It requires the support of a large number of the party’s voters, based on rules written by party elites. As a result, presidential candidates expect the party to support them once they win the nomination, even if some party elites would have preferred someone else.

Which is why the GOP opposition to Trump is unique – and even a little startling.

While moderate Republicans were unhappy with Barry Goldwater, much of their criticism emerged only after he lost the 1964 election in a landslide. In 1972, the Democrats for Nixon campaign attracted support from a few Democratic governors and mayors who openly supported Nixon over their party’s nominee, George McGovern.

But there are no other examples in which the congressional leadership and former presidents and presidential nominees openly rejected their party’s chosen presidential nominee.

Trump’s personality, his policy positions, his lack of political experience, and concerns about his judgment and leadership style make him a special case. Republican Party leaders may be worried about what a Trump candidacy will do to the GOP brand – both in Senate, House, and gubernatorial races in 2016, and after 2016 for the party as a whole. Based on current electoral predictions, those worries are legitimate.

We cannot expect politicians to blindly and fully support any candidate who wins their party’s nomination for elected office. For example, when former Ku Klux Klan Grand Wizard David Duke won the Republican gubernatorial nomination in Louisiana in 1991, Republican leaders rightfully rejected Duke en masse. They did so not only because of Duke’s toxic views, but also because they believed he could do major damage to the party as a whole if unchallenged.

Trump is clearly GOP voters’ choice, no matter what the elites think

But Trump – whatever his strengths and weaknesses – is not a random gubernatorial nominee who lucked into a party’s nomination.

Trump – as of now – has won 28 primaries and caucuses. In the popular vote, he has received 3 million votes more than Ted Cruz, his closest competitor. Trump will have a majority of delegates when the Republican National Convention meets in Cleveland in July.

Republican voters had quite a few options, with 17 candidates at the start of the primary season. The party’s voting base knew all along that the party leadership did not want Trump to be the nominee. But Republican voters chose Trump anyway.

By opposing Trump, Republican elites are sending a very strong signal — and taking a major risk

Their signal is that — to them — Trump is genuinely unacceptable. Every presidential candidate in the history of American elections has had major detractors within their party. But those detractors generally grit their teeth and endorse that candidate once they become the nominee. The fact that so many Republican elites are willing to openly reject Trump is a major indication of just how strongly they feel that he is unfit to be president.

The risk is that in sending this signal, these elites are undermining one of the key elements of the social contract underlying American political parties. Individual members — even speakers of the House or former presidents — cannot always get what they want in their party. For the party to function, members fall in line, if only because they expect other members to do the same when on the losing side.

By refusing to back Trump, Republican elites are rejecting a core institutional element of U.S. political parties — and dividing the GOP’s voting base. The result could be a complete fracturing of the party.

Boris Heersink is a PhD student in the Woodrow Wilson Department of Politics at the University of Virginia and a national fellow at the Miller Center. 

Jeffery A. Jenkins is a professor in the Woodrow Wilson Department of Politics at the University of Virginia and co-author of “Fighting for the Speakership: The House and the Rise of Party Government.”