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By Perry Bacon Jr.

When it comes to presidential nominees, the Republican Party often picks the next man up. Ronald Reagan, after losing the primary to Gerald Ford in 1976, was the GOP’s candidate in 1980. Reagan beat George H.W. Bush in 1980, but Bush was Reagan’s successor as the GOP nominee in 1988. Bob Dole (1988, 1996), John McCain (2000, 2008) and Mitt Romney (2008, 2012) all had to endure at least one loss before the party tapped them for the nomination.

Going by that pattern, it’s former Pennsylvania Sen. Rick Santorum’s turn now. He won 11 states in 2012 during the GOP primaries, emerging from a crowded field to finish in second to Romney. He has a decent electoral record, having won two terms as senator in a traditionally-blue state. And his positions on most issues fit within the GOP mainstream.

He will become the seventh Republican to formally announce his presidential campaign on Wednesday, joining a growing field seeking the GOP nomination next year. But the Republican Party is just not that into Rick Santorum.

In the early stages of the race, key party officials and donors have bypassed Santorum to back other candidates, particularly ex-Florida Gov. Jeb Bush, Florida Sen. Marco Rubio and Wisconsin Gov. Scott Walker. In Iowa, where Santorum won in 2012 after visiting all 99 counties, some of his key backers in 2012 are already defecting to other candidates, particularly Texas Sen. Ted Cruz and former Arkansas Gov. Mike Huckabee.

Santorum, despite finishing second in 2012, is so low in national polls right now that he may be excluded from a debate Fox News is hosting in August. Only the top 10 candidates in polls will be allowed to participate, and Santorum is currently outside of that group in some surveys. That low standing suggests he built little of a national following during this 2012 campaign.

Why aren’t Republicans galvanizing behind Santorum? Some of his challenges are long-standing. Santorum was defeated by a whopping 18 points in his 2006 Senate reelection bid, raising questions about his ability to win a presidential election.

He was an influential social conservative in Congress, leading an effort to pass a constitutional amendment to ban gay marriage. Santorum has downplayed issues like abortion and gay marriage recently, trying to emphasize his credentials on economic and national security issues, but more moderate Republicans embraced Romney in 2012 and will likely favor Bush or Rubio in 2016.

In 2012, Santorum outmaneuvered candidates like Herman Cain and Michele Bachmann on his way to becoming the favorite of evangelical and very conservative voters in many states. But the competition is much harder this time for the backing of the right-wing of the party.

In particular, Huckabee’s entrance into the 2016 campaign is a major barrier for Santorum, because his support base in 2008 mirrored Santorum’s in 2012.

In Iowa, Huckabee won 74 counties in 2008, compared to 63 for Santorum in 2012. But there was a huge amount of overlap: they each won 57 of the same counties.Of the 11 states Santorum won in 2012, Huckabee won five in 2008.

And while these differences are hard to measure with data, Huckabee is generally regarded as a more charismatic, engaging politician than Santorum.

Cruz, in looking to woo conservative Republicans, has an advantage over Huckabee and Santorum. Both Huckabee and Santorum have demonstrated in their previous presidential campaigns that they will struggle to get enough support from more moderate Republicans to win the nomination.Cruz at least can sell his potential, even if his odds of capturing votes outside of the party's right-wing are also low.

How could Santorum win? Santorum had done little organizing outside of Iowa in 2012, and he was ill-prepared to build on his victory there. His aides say they are now prepared for a longer, multi-state effort.

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Santorum almost certainly must win in Iowa again to be a real contender. His path would then require carrying other states in the middle of the country, such as Kansas and Missouri, that he won in 2012. But the question still remains: how could Santorum win the voters who backed Romney in 2012? A race between Santorum and Bush could have the same results as 2012, with Santorum winning small states, while Bush would be a favorite in large states with more moderate Republicans like Illinois and Florida.

Santorum, like Huckabee, is trying to avoid being defined only as the Christian conservative candidate and is appealing to low and moderate-income Republicans. The former senator is breaking with most of the other candidates in the GOP race by supporting an increase in the federal minimum wage.

But populism is a hard route to the GOP nomination. Santorum tried to cast Romney as an elitist in 2012, and it didn’t work. The Republican Party is fine with tapping more patrician politics, like George H.W. Bush, McCain and Romney.

Santorum, competing against a field with a weak front-runner (Romney) and few strong rivals in 2012, couldn't win. It's hard to seem him emerging against much strong opponents in this campaign.

NBC's Teddy Amenabar contributed to this story.