Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (Vt.) has said that he is "going to do everything in [his] power" to stop Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE from becoming president. Ironically, so far he is doing everything in his power to assure that presumptive GOP nominee Donald Trump becomes president.

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If Trump wins in November, then everything that Sanders believes in goes down the drain. The right will control the Supreme Court for another generation — and the court has been the most important agent of policy change in recent years. Trump will abrogate the Iran nuclear treaty, deport undocumented immigrants, begin racial profiling and impose immigration bans that violate America's traditions. He will attempt to lower taxes on the rich, repeal ObamaCare and eliminate healthcare coverage for millions of Americans. He will gut regulations on Wall Street, ends all efforts to control catastrophic climate change, and will promote the mining, drilling and use of fossil fuels. He will pursue a dangerous and reckless approach to the world.

Sanders also claims to be reforming the Democratic Party. However, any such reform would be largely worthless if Trump wins, since the Democrats would then be a shattered party. Republicans would control the Supreme Court, the presidency and the House, and likely retain control of the Senate. The GOP will dominate state and local governments as well.

By refusing to endorse Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE for president and threatening to take his campaign to the convention, Sanders is creating a condition that has almost always spelled defeat for the party holding the White House (currently the Democrats, of course). To find the last time that the incumbent White House party survived such a contest, it is necessary to go back to the 19th century, when James Garfield in 1880 won the presidency with an eyelash plurality of just one-tenth of 1 percent in the popular vote. If the party holding the White House cannot unite in defense of its incumbency, that sends a negative signal to voters, as well — not just Sanders's supporters, but all voters.

In protracting his campaign, Sanders is also tacitly encouraging his supporters to vote for a third-party candidate rather than the Democratic nominee. Third-party campaigns also are typically poison for the party holding the White House — for example, George Wallace in 1968, John Anderson in 1980, Ross Perot in 1992 and Ralph Nader in 2000.

The cumulative effect of Sanders's holdout can also be seen in the polls. Early polls do not predict the final outcome in November, but they can provide a snapshot of the current situation. Although Clinton is leading Trump, her percentages are still low, typically between 40 and 45 percent, with large numbers of undecided voters. In some polls, her lead over Trump nearly vanishes when third-party choices are included.

My formal system for predicting presidential election results, The Keys to the White House, confirms Sanders's negative impact on Democratic prospects. The keys are 13 true/false questions that diagnose prospects for incumbent parties to win the presidential vote. They have successfully predicted the results of all eight presidential elections from 1984 to 2012. If fewer than six keys are turned against the party holding the White House, it wins another term. If six or more keys are turned against it, the challenging party wins.

Currently, the incumbent Democrats are down four to five keys, with an additional two uncertain keys largely under Sanders's control: the internal party contest key and the significant third-party key. If these two keys fall against the Democrats, Trump will be the predicted winner. Even the loss of one additional key will make it more likely for Trump to prevail this fall.

It is not too late for Sanders to reverse course and keep the progressive dream alive. He is vastly closer to Clinton than to Trump on the issues. Sanders and Clinton voted together 93 percent of the time when they were both in the Senate, and Sanders has already gained the insertion of important progressive ideas in the Democratic platform. He didn't get all that he wanted, but after all, Clinton won the majority of delegates (both pledged and super) and the majority popular vote.

In the very near future, it is critical for Bernie Sanders to suspend his campaign and endorse Hillary Clinton — not just pro forma, but enthusiastically, and campaign on her behalf. Otherwise, the unthinkable for Sanders will become thinkable and America and the world will face the prospect of a Donald Trump presidency.

Lichtman is distinguished professor of history at American University in Washington.