If Rick Santorum wants to keep Mitt Romney from wrapping up the Republican nomination before the convention, he should encourage Newt Gingrich to stay in the race, not drop out.

Not everyone buys this theory, I admit. The doubters include Santorum — who keeps shoving Newt toward the exit — as well as quite a few leading conservatives, including Family Research Council head Tony Perkins and influential blogger Erick Erickson. They want to see a two-man contest between a “Massachusetts moderate” and a dyed-in-the-wool conservative.

I think they should be careful what they wish for. The “throw Newt from the train” people think that the math is on their side, but it isn’t.

It’s true that from the primaries and caucuses held so far, we know that the Romney vote is much smaller than the anti-Romney vote. In Ohio, for example, Romney managed a slim victory with 38 percent to Santorum’s 37 percent. But Gingrich, meanwhile, drew nearly 15 percent. Add those voters to Santorum’s, and Romney would have suffered a shattering defeat.

Santorum and Gingrich are both campaigning on the premise that Romney is not a genuine conservative. Both candidates draw support from self-described “very conservative” Republicans. Since Gingrich — who supposedly had a “Southern strategy” for winning the nomination — couldn’t even beat Santorum in Alabama and Mississippi, it’s clear who would have the better chance against Romney, mano a mano. Ergo, Newt, hasta la vista.

But this logic ignores the subtleties of the delegate math. Sorry to inflict a flurry of numbers, but here goes: To win the nomination, a candidate needs the support of 1,144 convention delegates. According to projections from the Associated Press, at this point Romney has 481 delegates, Santorum has 252, Gingrich has 128, and Ron Paul has 48.

By AP’s count, 1,356 delegates remain up for grabs in the remaining primaries and caucuses. That’s right: We haven’t even reached the halfway point of this seemingly endless slog to the convention in Tampa.

Both Santorum and Gingrich say that their goal is to keep Romney from reaching the magic number of 1,144 before the convention. After the first ballot, they would count on being able to persuade Romney’s delegates to abandon him in favor of a more authentic conservative.

This is a smart strategy, because — as the Romney campaign loves to point out — it is almost inconceivable that Santorum or Gingrich could win the nomination any other way. Santorum would have to win roughly two-thirds of all the delegates at stake in the remaining contests to secure the nomination before the convention. Gingrich would have to win even more. Not gonna happen.

Romney needs to win just half the remaining delegates. But that’s still no cakewalk, even with his vastly superior resources and organization.

The headline from Tuesday’s contests was that Santorum won in Alabama and Mississippi. But since delegates there and in most other GOP contests are awarded proportionally — and since there were also contests in Hawaii and American Samoa, where Santorum and Gingrich didn’t really compete — Romney ended the night having won 43 delegates, more than any other candidate.

But Santorum won 36 delegates and Gingrich won 24 — meaning that, while Romney increased his lead over the others, he fell short of winning half the delegates that were available. If he continues “winning” the delegate race at Tuesday’s pace, he will fail to wrap up the nomination before the convention.

What if Gingrich dropped out? It’s reasonable to assume that much of his support would go to Santorum, but not all of it.

My interpretation of what we’ve learned from exit polling so far is that Santorum’s voters tend to doubt Romney’s steadfastness on social issues, while Gingrich’s supporters tend to doubt that Romney is a true, small-government conservative. That’s an oversimplification, but I think it’s basically correct.

Gingrich voters who put less emphasis on social issues — or who doubt Santorum’s commitment to small-government principles — might well turn to Romney instead. Given the Romney campaign’s deep pockets, Santorum would face a blistering barrage of negative ads in every state. Legitimate questions about Santorum’s electability would be raised nonstop.

The Romney campaign is built for this kind of multi-theater battle. Santorum’s comparatively underfunded campaign is not. The most favorable field of battle for the anti-Romney insurgency would be a contested convention — and the most plausible way of getting there is for Gingrich to stay in the race and help keep Romney’s delegate count short of 1,144.

eugenerobinson@washpost.com