The race for the US presidency is heating up ahead of Super Tuesday - which sees 24 states vote for their preferred candidate.The results are expected to be a close call between Democratic hopefuls Hillary Clinton and Barack Obama while John McCain has emerged as the front-runner for the Republicans ahead of Mitt Romney.The presidential hopefuls have spent their weekend leading rallies and appearing on TV shows in an attempt to persuade any undecided voters to chose them.Mr Obama, who would be the first black US president, drafted in his celebrity friends to boost his campaign, with media mogul Oprah Winfrey and legendary musician Stevie Wonder both taking to the stage to show their support.Polls show Mrs Clinton and Mr Obama are locked in a near dead heat, despite the former First Lady's clear lead at the beginning of the campaign.Mr Obama held a slight lead in California, the biggest prize of all, where Mrs Clinton once led handily, and was virtually tied with her in New Jersey and Missouri - three of the states voting on Tuesday, according to a poll.While Mr Obama and Mrs Clinton vied to win the most delegates, they were also making the argument of being the most electable candidate to face Mr McCain in November.Mrs Clinton, a target of conservatives since being First Lady in the 1990s, said her record was well known and she had already weathered heated attacks while Mr Obama, a first-term senator from Illinois, is still an unknown quantity.In the Republican race, Arizona Senator, McCain has a double-digit lead on Mr Romney in New York, New Jersey and Missouri but narrowly trails the former Massachusetts governor in California.Up until now the race for the presidential candidate has been very unpredictable but after Super Tuesday it is likely there will finally be a clear leader from both political sides.© Independent Television News Limited 2008. All rights reserved.