Voters in the southern states of Alabama and Mississippi are casting ballots in Republican presidential primary contests Tuesday, with opinion polls showing the top three contenders in a tight race.

Mitt Romney, Rick Santorum and Newt Gingrich all campaigned for support in the predominantly conservative states Monday.

A win in either southern state would be a key for Romney. The former Massachusetts governor has won nearly 40 percent of the 1,144 delegates needed to secure the Republican party's nomination. But he has struggled to win the support of his party's conservatives base that has been drawn to his main rival, former U.S. senator Santorum.

In a boost for Romney, a new Washington Post-ABC News poll shows he leads President Barack Obama in a hypothetical election matchup, 49 to 47 percent. The poll, which has a margin of error of four percentage points, also shows Mr. Obama leading Santorum 49 to 46 percent.

Gingrich, a former U.S. House speaker, is staking his candidacy on wins in the southern U.S. after a victory last week in his home state of Georgia.

The only other state Gingrich has won is South Carolina, and failure to win at least one contest Tuesday could accelerate calls for him to drop out and let Santorum stand as Romney's sole conservative challenger.

The other Republican candidate, U.S. Representative Ron Paul has not won a nominating contest He has single digit support in Alabama and Mississippi.

The Pacific island state of Hawaii and the U.S. territory of American Samoa will hold caucus elections on Tuesday.

The Republican Party will formally nominate its presidential candidate at its convention in Tampa, Florida, in late August. The nominee will face Mr. Obama, a Democrat, in the November election.

###