At the moment, the Republican establishment is relevant to the presidential-nomination battle only as an epithet.

Less than two weeks from the Iowa caucus, the fight for the Republican nomination isn’t so much a vicious brawl between the grass roots and the establishment as it is a bitter struggle between traditional conservatism and populism that few could have foreseen.

Conservatism has always had a populist element, encapsulated by the oft-quoted William F. Buckley Jr. line that he would rather be governed by the first 2,000 names in the Boston phone book than by the Harvard faculty. But the populism was tethered to, and in the service of, an ideology of limited-government constitutionalism.

The fight between Ted Cruz and Donald Trump is over whether that connection will continue to exist, and whether the conservatism (as represented by Cruz) or the populism (as represented by Trump) will be ascendant. Cruz did all he could as long as possible to accommodate Trump, but now that the fight between them is out in the open, the differences are particularly stark.

Cruz is a rigorous constitutionalist. He’s devoted much of his career to defending the Constitution and has argued numerous cases before the Supreme Court. Trump has certainly heard of the Constitution, but he may know even less about it than he knows about the Bible.

Cruz is an advocate of limited government who is staking everything in Iowa on a principled opposition to the ethanol mandate. As a quasi-mercantilist and crony capitalist, Trump isn’t particularly bothered by the size of government and is happily touting his support for a bigger ethanol mandate.

Although Cruz is more flexible than his reputation suggests, he has the long baseline of consistency that you would expect from a genuine believer in a political philosophy. Trump has a few long-running themes and bugaboos, but has been all over the map on almost everything and sometimes will meander from one position to another within the same answer, in keeping with his lack of ideological anchor (and limited knowledge of policy).

The two have completely different political styles. Trump is instinctual and has a roguish charm, whereas Cruz is earnest and tightly disciplined. If almost everything about Trump is unconventional, Cruz is outwardly a very traditional politician.

Truth be told, the Texan is a prodigal son of the establishment. If you just looked at Cruz’s CV and had no idea about the mutual hatred between him and his party’s leadership, you’d figure he was the archetypical upwardly mobile Republican politician.

The irony of Cruz’s position now is that, despite all his outsider branding, he is not getting savaged by the establishment. Sure, fellow senators are looking for ways to shiv him, and Iowa Gov. Terry Branstad wants him to lose, but they aren’t his biggest worry.

It is Trump who calls him a hypocrite and a liar. It is Trump who is hitting him on his belated disclosure of a Goldman Sachs loan. It is Trump who says he’s a nasty guy and a maniac with a temperament problem. And it is Trump, of course, who constantly raises doubts about his eligibility to serve as president.

If you guessed a key event in the nomination fight would be the “othering” of the most potent tea-party conservative in the country by a billionaire businessman with a long trail of liberal positions and a history of praising President Barack Obama — well, then, you forecast the GOP race perfectly.

In short, Cruz is under assault from a segment of the anti-establishment, although Cruz takes every opportunity to portray himself as the victim of the machinations of dastardly political insiders. The reality is that the establishment is sitting on its hands, agonizing over whom it loathes least, Trump or Cruz, while the fight between populism and conservatism rages.

The battle for the soul of the GOP is now a battle for the soul of the right.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.