Whenever Mitt Romney starts to feel the heat from a potential challenger, it's time to roll out the endorsements.

National Journal says it's a well-worn and effective tactic for the Republican frontrunner.

Consider late September, the moment in the race at which Romney was the most vulnerable. Texas Gov. Rick Perry was challenging Romney for the lead in national polls, but perhaps more worrisome, the buzz around New Jersey Gov. Chris Christie was loud, and getting louder. Christie, it turned out, was actually considering making a late entrance into the race, setting up the possibility Romney would have to compete with a rising rock star within his own party, one who appealed to many of the same constituencies Romney did.

So Romney's campaign made clear that there wasn't room in the race for two Northeastern Republicans. On September 29, Romney announced support from 53 prominent Connecticut Republicans and from 61 Vermont Republicans. Christie said he wouldn't run the following week, on October 4 (Politico's Jonathan Martin made the connection between the endorsements and the clear message to Christie's team at the time).

Two months later, the media turned its attentions to Newt Gingrich, who, despite failing to put together any kind of national organization and neglecting even the most basic organizing in critical states like Iowa and New Hampshire, rocketed to the top of national polls. As Herman Cain collapsed following a series of setbacks in November, Gingrich began to take his place, leadingin national polls of Republican voters conducted by Quinnipiac and Opinion Research Corporation and reaching as high as 40 percent in an early December NBC News/Wall Street Journal poll.

Romney's team felt threatened again, and rolled out another roster of big-name endorsements. During the week of November 20, when Quinnipiac and CNN/Opinion Research Corporation showed Gingrich establishing his lead, Romney won public support from Sens. Kelly Ayotte, a big get in critical New Hampshire, and John Thune, the South Dakota Republican and rising star who's well-known among conservatives in neighboring Iowa.

Then, just five days after the NBC/WSJ poll showed Gingrich at 40 percent, Romney announced support from South Carolina Gov. Nikki Haley, another big name ahead of the January 21 Palmetto State primary.