It's been a good run for 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.): He got a lot further than anyone ever expected, brought some much-needed attention to some important issues, and probably pulled 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 somewhat to the left. He also tested Clinton's mettle and brought energy to the Democratic Party.

And now it's time for him to leave.

Sanders is an honorable man. He has consistently dedicated himself to helping and protecting the American people. But, in case you missed it, 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 is now the presumptive Republican nominee. This makes a major American political party one of the greatest threats to our security and the security of the world. The Republicans have chosen to nominate an anti-Muslim, anti-Mexican, anti-Chinese misogynist as their candidate for the highest office in the land and one of the most powerful positions on planet Earth.

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Flawed though she is, it's time for the Democrats to unite behind Clinton. If Sanders is sensible, he'll realize this and bring his campaign to an end. It's, thus far, been productive and effective. But, at this point, his continuing serves little purpose, and only undermines the effort to stop that egotistical, volatile bigot from entering the White House.

I don't think Trump will beat Clinton: The demographics don't favor him, his unfavorable ratings are too high, there's considerable disunion in his party and America has a history of usually turning away unpredictable and potentially dangerous candidates.

But even if there's only a 10 or 15 percent chance of Trump winning, that's too great a risk to take. The words of a president matter, and Trump is entirely capable of engulfing us in a trade war, alienating our allies and destroying the multiethnic fabric that is the very essence of our nation.

If Sanders presses on to the convention, as he has vowed to do, it will detract from Clinton's candidacy and distract her from the general election, where all of her energies should now be directed. Sanders will act as a shield for Trump from Clinton's fire, and will open her up to the criticism that she is too weak to clinch her party's nomination before the final primary.

Few doubt that Sanders is the genuine article and that he's never been interested in a job on K Street or profiting from $225,000 speeches. Few doubt his ideals.

But at what point does his quixotic journey come to an end? At what point does he decide that he's not going to be the Ralph Nader of this election? Nader had worthy and idealistic goals as well, but his candidacy accomplished nothing other than to help make George W. Bush president — an unmitigated disaster if ever there was one.

Sen. Sanders: You've fought important political and social battles with great courage for most of your life. And now the battle lines have been drawn: on the other side is a threat to the country as great as any over-leveraged bank and any Supreme Court campaign finance misjudgment combined. It's an egomaniac whose tax policies would be awful for our economy and the poor in particular, whose stance on global warming is to ignore it, and whose temperament could easily cause an international crisis at any moment.

It's time for you to be bold once again and to honorably and courageously bow out.

Rosenfeld is an educator and historian who has done work for Scribner, Macmillan and Newsweek.