Democrats fight for crucial votes

Mr Obama has closed Mrs Clinton's early lead in Ohio and Texas polls

Democrats Hillary Clinton and Barack Obama are fighting for votes on the last day of campaigning before contests in the big states of Texas and Ohio.

Mrs Clinton may face pressure to quit the race to be her party's presidential nominee in November if she fails to end Mr Obama's 11-contest winning streak.

She held early campaign stops in Ohio before heading to Texas, where Mr Obama is also campaigning hard.

Republican front-runner John McCain is expected to win his party's primaries.

Mr McCain may be hoping to gain enough of the 250 delegates at stake - in Texas, Ohio, Vermont and Rhode Island - to clinch his party's nomination at the national convention in September.

He currently has 1,014 delegates, according to the Associated Press, while Mr Huckabee has 257. A total of 1,191 delegates is needed to claim the nomination.

Security credentials

For the Democrats, a total of 370 delegates are at stake in Tuesday's four races, most of them in Texas and Ohio.

I just think that D-Day is Tuesday Bill Richardson New Mexico governor Clinton's steely resolve Democrats eye prize Last tango in Texas

Opinion polls suggest a close race.

Mrs Clinton held an early lead in Texas and Ohio, but Mr Obama has been gaining ground in both states.

Observers view the contests as critical for Mrs Clinton if she is to halt Mr Obama's momentum and stay in the race.

However, she has dismissed speculation that Tuesday's results could prove a make-or-break moment.

"I'm just getting warmed up," Mrs Clinton told reporters on Monday, saying she was looking forward to the contest in Ohio.

Clinton campaign strategist Mark Penn added: "This has been a rollercoaster race, and every time people try to foreclose it, the voters come back and say 'Just wait a minute, we want to have our say'.

During weekend rallies in the state, the New York senator focused her attacks on Mr Obama's foreign policy and national security experience, echoing a campaign advert asking who would respond better to a national emergency in the middle of the night.

Hillary Clinton is under pressure to halt Barack Obama's momentum

Mr Obama, senator for Illinois, countered by questioning Mrs Clinton's own credentials.

"What precise foreign policy experience is she claiming that makes her qualified to answer that phone call at 3am in the morning?" he asked at a town hall meeting in Ohio.

Speaking on Monday, he suggested that Mrs Clinton may have to reconsider her position if she does badly on Tuesday.

"If we do well in Texas and Ohio, I think the math is such where it's going to be hard for her to win the nomination," he said in an interview on the ABC network's Good Morning America programme.

"And they'll have to make a decision about how much longer they want to pursue it."

'Clear lead'

Observers point to signs that pressure may be mounting on Mrs Clinton from within her own party to stand down if she fares poorly in Tuesday's contests.

I think Hillary will win Ohio but by a very small margin, which may not be enough to save her campaign US voter Faith Barrow-Waheed, Cleveland, Ohio Voters' views: Texas and Ohio

New Mexico Governor Bill Richardson, who quit the race for the Democratic nomination in January, has not endorsed either candidate but has indicated he would like to see the battle resolved soon.

"I just think that D-Day is Tuesday," he told CBS television's Face the Nation programme on Sunday.

"Whoever has the most delegates after Tuesday, a clear lead, should be the nominee."

He said the Democrats had to be prepared to fight a "very strong John McCain" in November.

Mr Obama currently has 1,385 delegates to Mrs Clinton's 1,276, according to AP. A total of 2,025 is needed to secure the Democratic Party's nomination at its national convention in August.

Mrs Clinton has not won a primary or a caucus since the nationwide Super Tuesday contests on 5 February.