With victories in six of the 10 states on Super Tuesday March 6, Romney is urging supporters to stay the course.
"And so we will go, day by day, step by step, door by door, heart to heart," said Romney.
But the Republican race now heads to Alabama and Mississippi on Tuesday, March 13. And that offers new opportunities for Romney's rivals , former Pennsylvania senator Rick Santorum and former U.S. House speaker Newt Gingrich.
"We are staying in this race because I believe that it is going to be impossible for a moderate to win the general election," said Gingrich.
Romney continues to struggle to win over conservatives and likely will face setbacks in the upcoming southern primaries, said analyst Henry Olsen of the American Enterprise Institute.
"That means he is going to continue to slog through, picking up a delegate here, 20 delegates there, until he gets to some winner-take-all states that show up in June. But he is not going to put this away anytime soon," said Olsen.
Romney's opponents hope to slow him down enough to force a contested Republican convention in August, said ABC political strategist Matthew Dowd.
"By throwing a bunch of roadblocks in front, winning a bunch of states, keeping him from that number, and they hope to show how weak he is going into the convention, and basically turning to the convention and saying, 'you cannot nominate this guy,'" said Dowd.
The contentious and lengthy primary campaign worries former first lady Barbara Bush and other establishment Republicans.
"It's been, I think, the worst campaign that I have ever seen in my life. I just hate it. I hate the fact that people think compromise is a dirty word," she said.
Some Republicans do worry that the negative primary campaign has hurt Romney's chances against President Barack Obama in November, said Olsen.
"And he is demonstrating flaws as a candidate, a lack of adeptness, a coldness and a tone-deafness that is hurting him a little bit now, but people projecting forward are very worried that it is going to hurt him against an extremely well-financed and agile opponent," said Olsen.
Romney leads the delegate count with more than 400 at the moment, about a third of 1,144 needed to secure the Republican nomination.