Faced with a crucial test of his viability as a national candidate on Super Tuesday, Rick Santorum moved to shore up support in this state and in Tennessee among religious and very conservative voters, a bloc that has powered him to victories in several states, yet has been unable to coalesce around him and solidify his stature.

Tuesday’s 10-state matchup, in which 437 delegates are up for grabs, will provide another turning point in what has been a topsy-turvy race.

Although none of the four remaining candidates have signaled that Tuesday’s results would end their runs, a recent string of five back-to-back wins by Mitt Romney has strengthened his standing as his rivals continue to fight over the not-Romney title. Meanwhile, he rolls out endorsements.

On Sunday, two key congressional leaders from states that will vote Tuesday — House Majority Leader Eric Cantor of Virginia and Sen. Tom Coburn of Oklahoma — endorsed Romney.

Santorum said that he has not spent time courting endorsements and that the nods only prove that Romney is the candidate of the Republican establishment and Santorum the insurgent.

Santorum is expected to do well in Oklahoma and in Tennessee, where he attended a church service Sunday morning. He also greeted voters at Corky’s BBQ, a Memphis institution, where he ate ribs and potato salad with his family and supporters.

About 500 people showed up
at a rally in front of the state capitol, but protesters interrupted Santorum, forcing him to shout his attacks on President Obama and Romney, whom he criticized for enacting Massachusetts’s health-care law when he was governor.

For Santorum, a former senator from Pennsylvania, the most important contest will be in delegate-rich Ohio, where he has claimed some home-court advantage yet has not been able to get on every ballot in every congressional district.

Santorum’s central argument is that he can win in swing states such as Ohio, because he has done so before in Pennsylvania and because of his blue-collar roots and his focus on the manufacturing sector.

Yet a recent poll in Ohio
showed a surge of support for Romney and a neck-and-neck
contest. Santorum won 34 percent of likely Republican primary voters and Romney won 32 percent, according to a the NBC News-Marist survey.

Santorum shrugged aside the tightening polls in key Super
Tuesday states, which suggest a softening of support, calling the race for the Republican nomination a battle of “survival.”

And he began to hint that former House speaker Newt Gingrich (Ga.) should leave the race and allow Santorum to compete one on one with Romney.

“Ultimately for us to win this race, it’s going to have to narrow down to two. And I think that will happen eventually,” he said.

Gingrich, who had no public campaign events Sunday, touted a recent win in a conservative straw poll in Tennessee as proof that he remains a strong contender.

“I think there is a huge difference between Santorum and me,” Gingrich said on ABC’s “This Week” program. “Santorum has been historically a labor union senator from Pennsylvania. He voted against national right to work. So I think there’s some
pretty big policy differences there. And when you get out of the industrial states, I think it gets harder for Rick to put together a majority. So we’ll see how it goes next Tuesday.”

Gingrich is expected to win easily in Georgia, his home state.

For Romney, strong showings out West and in New England and Virginia, combined with a comeback win in Ohio, would add to
the growing sense of inevitability around his candidacy.

He leads in delegates, money and endorsements, and he has a polished and sprawling campaign apparatus.

On Sunday, he went into Gingrich country, stumping in Georgia and taking subtle swipes at his rival.

“I’m not going to come here and pander to you and say, ‘Here’s what your gasoline price will be if I do all those things,’ ” Romney said.

He added later, in a dig at his competitors: “The economy is what I do, it’s what I know, it’s what I’ve done. I haven’t just read about it. I haven’t just debated about it. I haven’t just talked about it on subcommittees. I’ve actually done it — started businesses, run businesses. I know how to do it.”

An aide said that wins in Georgia and Tennessee are unlikely, but that Romney is still likely to scoop up delegates in the South.

“I know that we can take
delegates out of there, and this
is a delegate contest now,” said
Romney campaign senior adviser Eric Fehrnstrom. “More important than winning this state or that state is achieving the requisite number of delegates you need to obtain the nomination. That’s what our focus is on.”

Romney will head into Tuesday with a significant advantage over Santorum, who will not be on the ballot in Virginia, an error he attributed to having “very limited resources early on.”

But the NBC News-Marist poll underscores a problem for Romney: The protracted and expensive primary fight has weakened his electability argument.

In a contest between Obama and Romney, the former Massachusetts governor trails by 12 percentage points in Ohio and 17 points in Virginia.

And prominent conservatives are beginning to question his viability against Obama.

“I think when this primary is over and people have had their heads knocked in by one another — that’s just the nature of a hard-fought campaign — we hit the reset button and the campaign begins anew with a different
opponent, and we’ll be able to draw sharp contrasts with the president and the president alone,” Fehrnstrom told reporters. “It’ll be a different race at that point and the numbers will move again.”

Although Santorum hauled in $9 million in February, mostly from small donors, he acknowledged that Romney’s spending
advantage has taken “a toll” on his poll numbers heading into Tuesday. But he insisted that he is in the campaign for the long haul.

“This is a game of survival,” he said.

Rucker reported from Snellville, Ga. Staff writer Rosalind S. Helderman in Memphis contributed to this report.