Record turnouts are expected in the U.S. presidential race as primary voters go to the polls in Texas, Ohio, Rhode Island and Vermont. The latest public-opinion polls show Democratic Senators Hillary Clinton and Barack Obama locked in very close contests in the delegate-rich states of Texas and Ohio. On the Republican side, Senator John McCain could secure his party's nomination with victories Tuesday over former Arkansas Governor Mike Huckabee.

Senator Hillary Clinton is fighting to keep her dream alive of becoming the first woman elected president of the United States. Polls show the race for the Democratic nomination too close to call in Texas, and give her a slight lead in Ohio, where she will await the results Tuesday night. Her husband, former President Bill Clinton, had asserted that she needs to win both Texas and Ohio to keep her campaign going.

But speaking from Texas on Comedy Central's The Daily Show late Monday, Clinton indicated she may fight on even if she does not win both states.

"It is still a very close contest in terms of the popular vote and the number of delegates," she said.

Senator Barack Obama, who is vying to become the first African-American U.S. president, has more committed delegates than Senator Clinton and is coming off 11 straight victories in primaries and caucuses since Super Tuesday on February 5. His campaign would love to deliver Clinton a "knockout blow" by winning both Texas and Ohio.

But campaigning Monday with his wife Michelle in Texas, Senator Obama asked his supporters not to take anything for granted.

"We have got to work in these last few hours. If you voted, then you have got to go find somebody who has not voted," he said. "If you already planned to caucus, you have got to round up five more friends to go to caucus with you."

Texans can choose to cast ballots in the Democratic or Republican primary, and thn attend a caucus held by that party after the polls close. Turnout has already been high in Texas, with an estimated 60 percent of voters casting their ballots in early balloting.

Polls show Obama with a strong lead in Vermont, and Clinton leading in Rhode Island.

Republicans are also voting in the same four primaries, with a total of 256 Republican delegates at stake. Senator John McCain only needs 177 additional delegates to clinch his party's nomination at the national convention in early September.

His chief remaining opponent, former Arkansas Governor Mike Huckabee has said he will stay in the race until someone obtains the required number of delegates. Senator McCain has already been focusing his energy on his Democratic rivals for the White House.