RUTHFULLY YOURS

After being in the minors for the last debate, Chris Christie graduated to the main stage of the Republican clash for the presidential nomination Tuesday night. And he made the most of it.

The governor of New Jersey was on the periphery of the debate’s dramatic fireworks — with Ted Cruz and Rand Paul trying to blow up Marco Rubio on foreign policy and immigration while Jeb Bush tried to elevate himself by going directly at Donald Trump as the “chaos candidate.”

So Christie played to his own strengths, talking tough on ISIS and terror while making the key point that the signal responsibility of the president is to keep the citizenry safe.

Christie’s future in the Republican race rests entirely on his ability either to win or place a very strong second in New Hampshire, which holds the first primary (after the Iowa caucus) Feb. 9.

He’s in the mix there, and his confident bearing and fluent presentation might well have the effect of solidifying his soft support in the state and causing others to take a renewed interest in him.

Certainly, this was the most important and substantive debate so far because the candidates finally began airing out their real differences. No one had yet laid a glove on Marco Rubio, but Tuesday night, Cruz and Paul went for his jugular on policy — and Rubio did everything he could to lay into them in response.

The divisions on foreign policy were stark. Rubio is the hawk of the race, advocating without apology for the use of ground troops against ISIS in Syria. Cruz talks about destroying ISIS in its territorial stronghold in Syria and Iraq from the air, which almost certainly cannot be done.

In response to Rubio, both Cruz and Paul essentially called him reckless and hungry for war — and tried to tie Rubio to the ideas of Barack Obama and Hillary Clinton.

Last night, Cruz openly began assuming some of Paul’s isolationist coloration — indeed, he made shocking use of the phrase “an America-first foreign policy.”

The term “America first” was the slogan of the isolationists who opposed American entry into World War II, as led by Charles Lindbergh. The intelligent and learned Cruz knows that, and did not use the term innocently.

Leave aside his adoption of that repugnant term. Cruz’s position at present is completely incoherent.

You can’t be an isolationist hawk, especially if you support drastic cuts in the defense budget, as Cruz does — but there’s no denying it may prove a tempting hedge for some Republicans.

On the other hand, Rand Paul’s isolationism has proved to be a ticket to Palookaville for him.

Rubio likely got the better of the foreign-policy fight, but he certainly got the worse of the exchanges over his stance on immigration.

Cruz ripped into the Florida senator for supporting a path to citizenship in the failed immigration-reform bill Rubio co-sponsored in 2013.

Rubio tried to get Cruz to admit he supported a legalization path that same year — Cruz said exactly that during a Senate hearing on May 21, 2013 — but Cruz skillfully used slippery Clintonian language to evade Rubio’s charge.

Rubio came out a bit bloodied. But that was inevitable. Immigration is his greatest weakness in the Republican primaries (and arguably his secret strength were he to be the party’s general-election candidate). It was bound to become a major discussion point at some point, and it became so last night.

The person with the most to gain from a successful Cruz assault on Rubio is Christie, since he is the only candidate with any momentum who might appeal to the GOP’s “somewhat conservative” voters.

These are the voters who will become the dominating force in the primaries once the race graduates from the small states and the South into the more populous and delegate-rich states.

For Cruz to get himself into a head-to-head as the undisputed conservative against the more moderate “establishment” choice, of course, Donald Trump will have to fade.

There was little reason to think last night’s debate will hasten such a thing. Trump was mostly subdued and only allowed himself to get rattled and nasty with Jeb Bush on two occasions. Otherwise, he probably did himself no harm and maybe a little good.

As for poor Jeb, he did the best he could — and the best he could isn’t going to do a thing to get his numbers up. He and his people are probably going to delude themselves into thinking his campaign renewed itself last night.

It didn’t. Jeb is toast.