The symbols of the Democratic and Republican parties are seen on display in Washington in 2008. (Karen Bleier/Agence France-Presse via Getty Images)

Eliot A. Cohen served in the Defense and State departments in the George H.W. Bush and George W. Bush administrations, respectively.

It’s over. Donald Trump, a man utterly unfit for the position by temperament, values and policy preferences, will be the Republican nominee for president. He will run against Hillary Clinton, who is easily the lesser evil but is trailed by clouds of scandal and misconduct and whose party’s left wing poses its own threats to liberties of speech, religion, enterprise and association.

It is time for a third candidate, and probably for a third party.

Some people will dismiss this notion as absurd. However, only those prescient enough to have forecast Trump’s success have the standing to certify impossibilities. If the Trump candidacy has blown up every other aspect of political conventional wisdom, why not this one?

Even if a third candidacy still yielded a Clinton victory, it would be worthwhile. It would, first, deny the Clinton campaign the illusion of a mandate from American voters who would have, en masse, turned out to reject Trump. If nothing else, a strong third-candidate vote would send her a message to govern from the center, rather than in deference to her party’s increasingly powerful left wing.

Republican front-runner Donald Trump says he can't wait to take on Democratic front-runner Hillary Clinton in the fall, but here are three reasons why he could lose a general election. (Sarah Parnass/The Washington Post)

A third candidate could lay the groundwork for a new political party. The Republican Party may right itself after this moral disaster, led by men and women of the caliber of House Speaker Paul D. Ryan (Wis.). But the sad truth is that although the speaker has the qualities of a statesman, two of his Republican predecessors have indicated that they would vote for Trump without qualms, while a third is too preoccupied with his upcoming jail term to say much of anything.

The Republican presidential defeat that likely impends will reflect an entirely appropriate national revulsion at the GOP candidate, whose personal record of chicanery and wild rhetoric of bigotry, misogyny and misplaced belligerence are without parallel in the modern history of either major party. It is entirely conceivable that the damage done will be permanent.

And in any case, the party of Lincoln is sick. The influence on it of ranting reality-television players, talk-show hucksters and monomaniacs of various stripes may not recede. The temper that led a supposedly responsible party of governance to repeatedly attempt to shut down the government may, in turn, shut it out of executive power for a long time.

A new, center-right party may be necessary — we cannot yet tell. If it is, the outlines of its platform are easy to anticipate: reverence for the Constitution; serious grappling with the domestic problems associated with economic opportunity for all, education and affordable health care; and commitment to the internationalist tradition of the post-World War II consensus. It would advocate a federal government that can energetically do the things it should, but would limit the role of unaccountable regulators and bureaucrats and push to states and local governments every function that is not clearly a duty of the federal government. Above all, it would be committed to liberty in every sphere of personal and public life.

A third candidate — and if it comes to that, a third party — must be led by a politician. The Great Republic does not require a man on horseback to rescue it, despite the arguments that some have made for drafting a retired general. Senior military officers usually make dreadful politicians, and besides, politics is an art — a respectable art, despite what too many Americans think — with unique skills and aptitudes. People with such skills exist, including Mitt Romney. The question is whether one of them will step forward.

One of them should, for this final reason: to keep conservative consciences clean. To vote for Clinton is to sacrifice standards and endorse policies and conduct no conservative should; not to vote at all is an escape, not a civic deed.

Admittedly, this may be a losing cause. But a losing cause is not necessarily a futile one. John Quincy Adams fighting slavery in the 1830s and 1840s and Wendell Willkie running on an internationalist platform in 1940 proved that. A Trump candidacy is a disgrace and has indeed already damaged us at home and abroad, but the longer-term question is larger than one demagogue, dangerous though he is. It is whether the cause of free, limited and constitutional government will have someone to speak for it and to represent it now and for decades to come.

The hour is late, the task is urgent, and the cause is great. Let us hope that some politicians will summon the courage that their country requires, and act.