The South's turn: Romney, Santorum, Gingrich vie

Caucuses in Hawaii were also on the calendar in the race to pick an opponent to President Barack Obama this fall.

There were 107 Republican National Convention delegates at stake, 47 in Alabama, 37 in Mississippi, 17 in Hawaii and six more in caucuses in American Samoa.

Each of the three leading contenders faced a different challenge in Alabama and Mississippi, where heavy television advertising was evidence of the states' unaccustomed significance deep in the nominating campaign.

Gingrich struggled for political survival, Romney sought a strong showing to silence his critics and Santorum hoped to emerge at last as the chief conservative rival to the front-runner.

Rep. Ron Paul, the fourth contender, made little effort in the states on the day's ballot.

Evangelicals played an outsized role in both Southern primaries, underscoring the challenge to Romney. In Mississippi and Alabama, roughly four in five voters surveyed as they left their polling places said they were born again or evangelical.

Those voters have been reluctant to rally to Romney's side in the primaries and caucuses to date. His best showing in a heavily contested primary so far was 38 percent in Florida.

As has been true in earlier primaries, the economy was the most important issue to voters, and an ability to defeat Obama the most important quality when it came time to pick a candidate.

The exit polls were based on interviews with 1,024 voters as they left 30 randomly selected polling places around Alabama, and with 1,102 Mississippi voters from 30 sites. Each survey had a margin of sampling error of plus or minus 4 percentage points.

The Southern showdown came as new polling showed a decline in Obama's approval ratings — a reversal amid escalating gasoline prices and turbulence in the Middle East.

The day began with Romney leading the delegate competition by far in The Associated Press count, with 454 of the 1,144 needed to win the nomination. Santorum had 217, Gingrich 107 and Paul 47.

That gave the former Massachusetts governor more than his rivals combined. And while Santorum in particular challenges the mathematical projections, Romney is amassing delegates at a rate that puts him on track to clinch control of nomination before the convention next summer.

Romney, campaigning in Missouri, took exception to a television commercial airing in both Southern states and said Santorum "is at the desperate end of his campaign." The commercial was backed by a super PAC that supports the former senator, not by him.

Santorum's camp had earlier issued a memo that dismissed as fuzzy math Romney's claim that he is on track to amass a delegate majority. "Simply put, time is on our side," it said.