Voters in Alabama and Mississippi are casting ballots in Republican presidential primaries Tuesday, with opinion polls showing tight races in both states.

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

The results of the races are important to each of the candidates.

Romney is hoping for victories to propel him forward and prove he can win over very conservative and evangelical Christian Republicans, who have been drawn to his main rival, former U.S. senator Santorum.

Santorum wants to knock Gingrich out of the race to stand as the sole conservative challenger to Romney.

Gingrich, a former U.S. House speaker, has focused his efforts on the southern vote and is hoping victory on Tuesday will make him the comeback favorite for the nomination. Otherwise, he could face increased calls to drop out.

Romney, a former Massachusetts governor, is far ahead of the others in the delegate count, winning nearly 40 percent of the 1,144 needed to secure the party's nomination.

A new Washington Post-ABC News poll shows he leads U.S. President Barack Obama in a hypothetical election matchup , while Santorum would be in a competitive race, three points behind Mr. Obama, if the election were held now.

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 are holding 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.