The Fox Business debate made clear that the Republican primary is Donald Trump's race to lose. It's not only that Trump continues his months-long polling lead and the debate won't do anything to change that. It's that Trump's considerable political skills were on display Thursday evening. Provocative, gauche, funny, emphatic, and fearless, Trump doesn't back down when the crowd boos him, he holds his own against more polished opponents, and he has identified and exploited the anger of many Republican and independent voters.

But Trump has done something more. He's become the focal point of the 2016 campaign in a way no other candidate of either party has. Fair or not, it's Trump's statements and policy declarations that become fodder for debate questions and force his GOP rivals to respond or adjust. Look at how he's changed the tenor of Marco Rubio's campaign from being sunny and future-oriented to being exasperated at the state of the country under Barack Obama. Trump's suggestion that Democrats would challenge the legality of Ted Cruz's status as a "natural born citizen" forced Cruz to go on the attack—something Cruz did not anticipate having to do this early.

Cruz, the champion debater, easily bested Trump on the citizenship question. But it's hard to say how the GOP electorate will respond to their other exchanges. When saying Trump represented "New York values," Cruz was acting as though the only Republicans who vote in primaries are very conservative and the only region that matters to the GOP is the south. He may have helped himself in Iowa, but also harmed his campaign in other states in a contest where every delegate counts.

Consider for a moment how remarkable it is that the two leaders in the race are outsider candidates despised by the GOP establishment. The traditional pattern would be for a conservative candidate, in this case Cruz, to go up against a center-right candidate more friendly to Wall Street and the Chamber of Commerce. Not this year. While all of the candidates have their strengths, it was clear on Thursday night that the real fight is between Trump and Cruz. The exchange between Rubio and Chris Christie over the New Jersey governor's record was fascinating. But it seemed superfluous, as though it were part of another primary entirely.

For much of 2015, the question was, Is Trump serious? The past week should have settled that in the affirmative. So the question becomes: Who will stop Trump? Ted Cruz? Marco Rubio? Chris Christie? Hillary Clinton? And what happens if, as I suspect, the Trump phenomenon cannot be stopped?