PORTLAND, Maine -- A day after Mitt Romney regained some momentum in the Republican presidential contest, his rival Rick Santorum went on the attack, calling the front-runner "desperate" while promising to compete aggressively to win the state where Romney grew up.

Santorum said Sunday he could do "exceptionally well" in Michigan, where Romney's father served as governor. The Midwestern state and Arizona host Republican presidential nominating contests on Feb. 28.

"We're going to spend a lot of time in Michigan and Arizona, and those are up next. And that's where we've really been focusing on," Santorum told ABC's "This Week." He suggested a strong showing in those contests would make the presidential contest "a two-man race," dismissing rivals Newt Gingrich and Ron Paul.

Santorum shrugged off his third-place finish Saturday in caucuses in Maine, where he didn't actively compete, as well as his second-place finish in a straw poll of conservative activists. Romney has been painting Santorum as a long-time Washington insider who pursued home-state projects. Santorum on Sunday described Romney's recent criticism as "desperate."

"You reach a point where desperate people do desperate things," said Santorum, who represented Pennsylvania for 16 years in Congress, first in the House and then in the Senate.

But questions about Romney's durability as his party's presumed front-runner persist.

Maine GOP officials declared Romney the winner of the caucuses Saturday. The results ended a three-state losing streak to Santorum, who swept Colorado, Minnesota and Missouri on Tuesday.

After six contests in 14 days, the candidates have 17 days to raise cash and bolster their organizations for what's shaping up to be a slog to the Republican nomination.