(Image Credit: AP Photo)

Momentum is the darling of every political campaign. Advisers and operatives are paid millions each election season to help their bosses seize it. But the effects cut both ways. After a few brief surges, Rick Santorum's candidacy has finally, and perhaps fatally, caught the bad strain - the one that looks like an unbreakable, interminable death spiral.

If only he knew it.

Here now, for the former senator and so many other politicians who couldn't see it was time to kindly leave the stage, is a simple guide to knowing when your campaign has run its course:

Your opponent has stopped mentioning you in speeches.

Mitt Romney said a lot of things Tuesday night after completing his sweep of Wisconsin, Maryland and Washington, D.C., but at no point did he make any competitive reference to "Rick Santorum." The frontrunner has been brushing off his opponent for some time now, but this wasn't gamesmanship or your standard election season head-fake. Romney's "pivot" was real.

Another bad sign for Santorum: President Obama has finally singled out an opponent… and it's not Rick Santorum. The president Tuesday referred to Romney by name for the first time. His campaign also did it in a new commercial.

Said Romney: "The president put an ad out talking about gasoline prices and how high they are and guess who he blamed? Me."

(Sounds a bit like bragging.)

Your bowling score on a given day gets more publicity than anything else you said or did.

On Monday,  Rick Santorum beat a little girl bowling, 152-150. It was a close game covered (closely) by the national media. Surely, the candidate would blame the traveling press for eschewing whatever substantive statements he might have made before, after, or during the match, but the fact remains: 152-150. For most people who are still paying attention to "Santorum '12," that was the day's most buzzworthy clip, his visit to the  "birthplace of the Republican Party" placing a distant second.

You are repeatedly likening your campaign to Ronald Reagan's … from 1976. 

Every Republican candidate plays the Reagan card, but a closer look at Santorum's latest round of footsie with The Gipper's ghost deserves a second look. Or maybe a first. For weeks, Santorum likened his campaign to Reagan's 1976 "insurgent" run, only recently acknowledging he realizes Reagan lost that race.

You are banking on a state that voted you out of office by an 18-point margin six years ago to save your campaign.

Santorum gave his Tuesday night concession speech/rally cry from Mars, Pa. The former Keystone State senator made his way there ahead of the bad news from Wisconsin. By his own admission, Santorum needs to win his home state to keep his White House dream alive. But not-so-distant history suggests that while he can go home again, he probably shouldn't expect a groundswell of support now that he's arrived.

Unlike Newt Gingrich, who stoppered one of his original slides with a big win in Georgia (where the speaker won seven straight congressional elections before resigning after the GOP's disastrous nationwide performance in 1998) , Santorum did not leave Pennsylvania on good terms. Voters there denied him a third Senate term by an 18-point margin in 2006. And that's not just down to a Democratic surge. Santorum suffered with the Republican base, which was - and might still be - unhappy with him for backing former Sen. Arlen Specter over the more conservative Pat Toomey during a rough 2004 primary showdown.

The Duggar family clogs the room at some of your events.

There are lots of reasons that can complicate a candidate's decision to leave a race. There is debt, previously scheduled public appearances and obligations, and the livelihood of your staff, to name a few. Also, in Santorum's case, you might not want to let down  the Duggars. Members of the reality TV family (TLC's "19 Kids and Counting") have been by Santorum's side for months, their presence alone enough to pack a small town hall.

Even the people who used to like you, and maybe still do, are voting for the other guy.

ABC News polling director Gary Langer analyzed the exit polls Tuesday night and found that "Santorum, the darling of very conservative voters in the South, only about split them with Romney in Wisconsin. … Romney ran competitively among groups he's lost in states farther to the south. He was close to Santorum even among evangelicals and very conservatives in Wisconsin, two groups that had lifted Santorum to Southern victories. And there were fewer of them: Fewer than four in 10 voters either in Maryland or Wisconsin described themselves as evangelicals, compared with 61 percent in Louisiana and an average of 53 percent in all GOP primaries to date."

Simply stated, the same people who voted Santorum in the past have now either turned to Romney or given up the cause altogether.

The party's national committee is raising money for and with your opponent - the same opponent who outspent you four-to-one in the last primary state.

The Republican National Committee has revealed it will begin  raising money jointly with the Romney campaign. That cash will not and cannot be used against primary opponents, but it will be solicited in Romney's name.

Endorsements! They rarely swing a race, but they can seal it.

There are three kinds of endorsements:

The Big Lift: Ted Kennedy's enthusiastic and relatively early support for candidate Obama in 2007 put a serious dent in Hillary Clinton's inevitability narrative. It was a politically brave decision by the late Massachusetts senator and provided the Obama campaign with some needed establishment cred.

The Bandwagon Hop: These are often a bit humorous looking back. Al Gore embracing Howard Dean during the 2004 Democratic primary race comes to mind. Dean was an early frontrunner and Gore hopped aboard with some theater at the height of the former Vermont governor's popularity. Alas, like most of this vintage, the relationship didn't age well. Dean faded once the voting started. This year, Ohio Attorney General and former U.S. Sen. Mike DeWine  jumped ship from Romney to Santorum in mid-February, days after Santorum pulled off a sweep of his own, winning Missouri, Minnesota and Colorado.

The Closer: Nothing said "game over" in the Republican race like the photo op following former President George H.W. Bush's official official endorsement of Romney. The Bush clan tends to have its way when it comes to GOP political jockeying and what better way to preserve that record than to join the game with the result already in the bag.

Also signing on in the past weeks: Former Florida Gov. Jeb Bush, Florida Sen. Marco Rubio, and Rep. Paul Ryan from Wisconsin. They waited, but made no mistake.

You are caught cussing at - or in response to - reporters on a rope line.

There's nothing new or particularly terrible about politicians and their aides saying mean things to reporters. This is a high-stakes business and people get emotional. But the general idea is to keep it off camera, off the rope line and off national TV. Santorum's run-in with a New York Times reporter, his young daughter looking on in some shock by his side, did not suggest a candidate on the up.