It's one of the greatest examples of “careful what you wish for” in political history: President Obama is going to be replaced by the kind of Republican he's always said he wanted.

For the entirety of his presidency, Mr. Obama has insisted that he is a pragmatist, not an ideologue. Indeed, he seems to think that ideology is a dirty word. “What is required,” Mr. Obama declared the day before his first inauguration, “is a new declaration of independence, not just in our nation, but in our own lives — from ideology and small thinking, prejudice and bigotry — an appeal not to our easy instincts but to our better angels.”

As a confessed ideologue, I've always taken offense at the suggestion that ideology — i.e., a fixed set of principles — deserves to be listed alongside prejudice, bigotry and small thinking. Moreover, as a conservative, I've always found laughable the idea that Mr. Obama is not an ideologue.

But when Donald Trump says he's a pragmatist, it's no laughing matter. Not since Richard Nixon have we had a president (or president-elect) less committed, or beholden, to a fixed ideological program.

Going into the GOP primaries, the conventional wisdom held that the winner of the contest would be the candidate who displayed the most ideological purity. Instead the brass ring went to the contender with the least.

“No, it's not going to be the Trump doctrine,” Mr. Trump said in April. “Because in life, you have to be flexible. You have to have flexibility. You have to change. You know, you may say one thing and then the following year you want to change it, because circumstances are different.”

A few days later, he told his supporters in California, “Folks, I'm a conservative, but at this point, who cares? We got to straighten out the country.”

His surrogates echoed the sentiment. Investor Carl Icahn assured voters that “Donald is a pragmatist. He's going to do what's needed for this economy.”

Hedge fund mogul Anthony Scaramucci wrote in the Wall Street Journal: “What elitists misinterpret as uneven principles, entrepreneurs understand as adaptability. ... Mr. Trump would be the greatest pragmatist and deal maker Washington has ever seen.”

The closest Mr. Trump comes to a rigid set of political principles is on the issue of trade. He has been making the same protectionist arguments about trade for more than 30 years. And despite the fact that the GOP has, at least rhetorically, been a party of free trade since Ronald Reagan, Mr. Trump seems to have won that argument in a rout. No doubt there are Republicans who disagree with Mr. Trump on trade, but for the most part they're keeping their opposition to themselves.

Mr. Obama came into office wanting to be a transformative president. He almost certainly failed — many of his prize accomplishments likely won't survive the next GOP Congress. And even as he argued against partisanship, and advanced the idea that a president can, nay must, decide every issue on a case-by-case basis, he always pushed a liberal agenda.

Mr. Trump, though, really might try the case-by-case approach, which we'll soon find is more disorienting than refreshing. His “flexibility” on numerous issues — infrastructure, entitlements, industrial policy, day care and who knows what else in the years to come — means we won't know what to expect.

For good or ill, then, Mr. Trump could be the “transformative” president Mr. Obama always wanted to be — the president who gets us past partisan ideology by doing away with principle.

One can already hear the ideological supports of both parties groaning under the weight of Mr. Trump's pragmatism. If one party collapses as a result, both will likely topple over. What replaces them is anyone's guess, but no one will deny that a transformation took place.

Jonah Goldberg is a fellow at the American Enterprise Institute and a senior editor of National Review. His email is goldbergcolumn@gmail.com. Twitter: @JonahNRO.