DERRY, N.H. — Rick Santorum has it all planned out.

“Several races down the road, this field will narrow and it will be a one-on-one race – Mitt Romney against Rick Santorum – and we’ll win this race,” the Pennsylvania senator said at an event here Monday afternoon.

One major problem: Ron Paul.

We wrote before on this blog about how Paul was Romney’s “best friend” in the Iowa caucuses, because he was instrumental in chopping down Newt Gingrich, while not really threatening Romney to become the GOP nominee.

Republican presidential candidate Rep. Ron Paul (R-Texas) holds a town hall meeting with employees of Timberland LLC on Monday in Stratham, N.H. (Photo by Chip Somodevilla/Getty Images)

Santorum and the other GOP presidential candidates all seem to be shooting for the path to victory he described above – i.e. survive long enough to get Romney mano-a-mano and try to beat him then.

The problem with that theory is that Paul ain’t going anywhere. His campaign has acknowledged it is shooting for a brokered convention (more on this idea later today) and is already discussing its strategy for after Florida’s primary on Jan. 31.

And he can do it – or try, at least – because he’s got a very reliable base that will continue to fund his campaign even as it becomes clear he’s not going to win.

So why does that matter?

With Paul still in the race, it becomes much harder for whatever other non-Romney candidate might emerge, because the anti-Romney vote is suddenly split in two.

If Paul can continue to take 15 or 20 percent of the vote — or even just 10 percent — in these contests, the threshold of victory for Romney in that three-way race is much lower than it would be in a head-to-head race.

And the more Romney keeps finishing first, the more it will be clear that he’s the presumptive nominee, which will probably only increase the margin of his victories. At that point, it will likely be impossible for the non-Romney candidate to continue to fund and run a real campaign.

Remember 2008, when Mike Huckabee seemed to be sticking around even as the race was pretty clearly coming down to Romney and Sen. John McCain (R-Ariz.)? Huckabee’s continued presence in that race — particularly on Super Tuesday — similarly split the anti-McCain vote and probably ruined any chance Romney had of overtaking the frontrunner.

This time, Romney could very well be on the winning side of that equation.