Sen. John McCain (R-Ariz.) met up with Mitt Romney at a town hall in New Hampshire today and endorsed his former rival. McCain echoed Romney’s electability argument: “The time has arrived for Republicans to choose a presidential nominee; a new standard-bearer who has the ability and determination to defeat President Obama and the strongest commitment to returning America to prosperity and defending our interests and values overseas. . . . Governor Romney offers us the common-sense reforms of government policy that are necessary to turn around our economy. His record of accomplishment in government and business are a testament to his leadership abilities. His commitment to a strong defense and principled diplomacy will earn the world’s respect for American leadership.” That’s a message friendly both to New Hampshire voters and general-election voters, one that is heavy on economics and doesn’t dwell too much on social issues.

McCain is no darling of the right, but at this point Romney is concerned about two things: winning convincingly in New Hampshire, and making sure he faces in South Carolina a divided field of conservatives and no real competition for moderate voters. In other words, he wants to bury Jon Huntsman in New Hampshire, and if possible, to slow down the Rick Santorum train.

The first of these aims seems doable. The Suffolk University tracking poll of likely voters in New Hampshire’s primary reports: “Huntsman dropped three points, from 10 percent yesterday to 7 percent today. [Ron] Paul dropped two points, from 16 percent to 14 percent, despite a third-place finish in Iowa last night. The poll showed Romney maintaining his lead with 43 percent of the vote, followed by Paul (14 percent), Newt Gingrich (9 percent), Jon Huntsman (7 percent), while Rick Santorum (6 percent), fresh off an impressive Iowa Caucus second-place showing, managed to climb another point and is now within striking distance of third place.”

Remember that only candidates polling at least 7 percent nationally or in the top four slots in Iowa or New Hampshire get into the South Carolina debate. The candidate who finishes last in New Hampshire could lose a debate slot and be out of the running thereafter.

It’s easy to forget that one primary does impact the next and that, as candidates disappear, the electorate rearranges itself. In the case of New Hampshire, Santorum will certainly move up and Paul, in all likelihood, can retain a good chunk of his support. Likewise, candidates who finish at the back of the pack in New Hampshire after Iowa will have trouble convincing South Carolina voters (who famously like to pick winners) that they are credible contenders.

In that regard both Romney and Santorum are playing it smartly. They both understand that the Jan. 21 South Carolina primary isn’t a life raft; It’s a rung on a ladder. By climbing one step at a time, you make each subsequent step easier, building confidence and gaining money as you go. Those who don’t plan on making an effort in New Hampshire may find themselves in desperate straits by Jan. 22.