The Republican Party came to life as the bastion of “Free Soil, Free Labor, Free Speech, Free Men.” It was a reformist party dedicated to stopping the spread of slavery and to fighting a “Slave Power” its founders saw as undermining free institutions.

The new political organization grew out of the old Whigs and reflected the faith that Henry Clay and his admirer Abraham Lincoln had in the federal government’s ability to invest in fostering economic growth and expanding educational opportunity. Its partisans embodied what John C. Calhoun, slavery’s chief ideological defender, described disdainfully as “the national impulse.” It was, in fact, a good impulse.

But the Republicans who held their first national convention 160 years ago were more than just northern Whigs. Their ranks also included many former Democrats who shared a fervor for the anti-slavery cause and helped take some of the Whiggish, elitist edge off this ingathering of idealists and practical politicians.

“The admixture of Whig and Democratic politics inside the Republican Party,” writes the historian Sean Wilentz in “The Politicians & The Egalitarians,” his recently published book, “created a forthright democratic nationalism, emboldening the federal government, for a time, at once to stimulate economic development and broaden its benefits.”

The Republicans descending on Cleveland would thus have every right to insist that all Americans owe a large debt to the GOP. We are a better, freer and more prosperous nation because their party was born.

Of course it would be historically naive to pretend that time has stood still since 1856. To do so would mean ignoring that the South, which hated the original Republicans, is now the dominant force in the party. It would involve being blind to the way in which our two great political parties have switched sides in how they view the capacity of our federal government to promote a more inclusive prosperity.

It would be equally untrue to history to claim that the nativism of Donald Trump is alien to the party. On the contrary, the anti-immigrant, anti-Catholic Know-Nothings were an important force in early Republicanism, and the party embraced opposition to newcomers at various points in subsequent eras.

Nonetheless, Republicans who are not in the least progressive have reason to mourn what is likely to come to pass this week: the transformation of the Party of Lincoln and Dwight Eisenhower into the Party of Trump. Some are bravely resisting this outcome to the end — and good luck to them. A fair number of leading Republicans have stated flatly that they will never vote for Trump. Their devotion to principle and integrity will be remembered.

But so many others in the party have found ways of rationalizing support for a man who plainly does not take governing, policy or even what he says from one day to the next seriously. It is comical but also embarrassing to watch politicians and consultants fall all over themselves to declare that Trump is “maturing” because every once in a while, he reads partisan talking points off a teleprompter. This is seen as a great advance over the normal Trump whose free association rants refer to his opponents as “lyin’,” “crooked,” “sad,” “weak,” “low energy” and — in the very special case of Sen. Elizabeth Warren — “Pocahontas.”

Liberals have long complained about conservatives “dog whistling” appeals to racial animosity. But hypocrisy really is the tribute vice pays to virtue and so it does mark a decline in simple decency that Trump has shouted out his prejudices openly: falsely claiming that Barack Obama, our first African-American president, was not born in the United States; railing against Mexican immigrants as “rapists”; and calling for “a total and complete shutdown of Muslims entering the United States.”

And a party that helped build popular support for internationalism after World War II is about to turn to a man whose foreign policy pronouncements defy coherence. He’s not even consistent in supporting noninterventionism or protectionism, both of which are part of a historically legitimate Republican tradition. He substitutes bullying for choosing, bluster for strength.

Many Republicans oppose Trump because they see him as the one candidate most likely to lose to Hillary Clinton. But others fear something worse: a Trump victory. They know that his presidency would represent a grave danger to the republic, a repudiation of the most noble Republican aspirations, and the end of their party as a serious vehicle for governance. The GOP can survive a Trump defeat. It will never get over being permanently defined by his politics of flippant brutality.

E.J. Dionne Jr. is a columnist with The Washington Post. His email address is: ejdionne@washpost.com. Twitter: @EJDionne.