( dpa )- Voters in the large states of Ohio and Texas head to the polls Tuesday to weigh in on presidential nominations, with all eyes on the contest between Senators Hillary Clinton and Barack Obama as they battle it out for the Democratic Party nod.
Voters in the smaller states of Rhode Island and Vermont will also cast ballots, while Republican Senator John McCain is expected to tighten an already vise-like grip on his party's nomination.
Obama and Clinton have dominated front pages and top broadcast billing with their every step and stumble under a microscope in these late stages of their unprecedentedly close intra-party race.
Clinton's charges in a debate that she was subjected to more media scrutiny prompted one of the nation's favourite political satire shows, Saturday Night Live, to portray famous television journalists fawning over Obama.
Clinton, a former first lady and two-term US senator, has been hammering the issue of national security and Obama's lack of experience.
A television ad from Clinton's campaign shows sleeping children, then asks whom viewers would want to be in the White House when the phone rings at 3 am.
Within hours, Obama had a similar ad on the air, showing uncannily similar sleeping children, asking a similar question, and answering that the people want someone who would not have voted to give US President George W Bush authority to invade Iraq.
Obama's criticism of Clinton has centred on her 2002 vote to authorize the Iraq war, coupled with his argument that she's too much of a political insider in a country that wants change.
In turn, Obama has been criticized by Clinton - and even from the White House - for his declared willingness to meet with leaders of hostile nations.
"This is going to be a campaign about national security," Clinton said at a recent campaign stop, her voice hoarse. "If Senator Obama doesn't want to debate me about national security, how is he going to debate (McCain) about national security?"
Obama has been outdistancing Clinton in raising campaign money and has increasingly outspent her. He has spent about 15.3 million dollars on TV advertising since early February in Texas and Ohio, compared to Clinton's 8 million dollars in ads for Tuesday's major states, The New York Times reported.
If Clinton fails to win both Ohio and Texas, pundits believe Obama will seal the Democratic nomination on Tuesday.
Obama has won the last 10 state contests, giving him a significant lead in the battle for delegates to the centre-left Democratic Party's nominating convention in August. Even former president Bill Clinton has said that his wife must win both Ohio and Texas for her campaign to remain viable.
Obama and Clinton have crisscrossed both Texas and Ohio over the last two weeks as they traded accusations over trade, health care policy and who can bring change to Washington, and recent polls have shown that Tuesday's result is anyone's guess. Obama holds a slight edge over Clinton in Texas while Clinton has a small lead in Ohio.
The smaller, north-eastern states of Vermont and Rhode Island are also set to vote on Tuesday.