A long time ago, in the era we now know as B.T. (Before Trump), it was possible to envision a Republican primary campaign that would be a real contest of ideas, a clash of genuine policy visions — and therefore different from the empty I’m-not-Obama, no I’m-not-Obama contest of 2012. My favored scenario would have pitted Marco Rubio against Rand Paul: The former representing a reform-minded conservatism in domestic policy and a hawkish internationalism abroad; the latter representing a more libertarian domestic agenda and a noninterventionist posture overseas.

We’ve had tastes, in the Republican debates, of what that contest would have looked like, mostly when Paul has sniped at Rubio from the corner of the stage. But the Kentucky senator’s moment came and went a year ago; in 2015, like so many others, he’s been Trumped.

Yet the possibility of a real clash of ideas hasn’t gone the way of Paul’s campaign. Instead, if the race came down to the three men currently leading in the national polls, Republican primary voters would be facing their most ideologically consequential choice since 1980. Unlike many G.O.P. campaigns, in which terms like “establishment” and “populist” are mostly about affect and rhetoric, this time the Republican front-runners offer three very different visions for the future of the party.

Read the article “The GOP at a Crossroads” on nytimes.com.