Frantic race around the US as rivals scramble for Super Tuesday votes

America is now a passing blur for the four remaining candidates in the race for the White House.

In foul winter weather, that dumped inches of snow and freezing rain on much of the country, the candidates soldiered on yesterday, clocking up air miles and running down the clock before Super Tuesday, when millions of voters in 24 states have their say. Democrats abroad and voters in American Samoa will also take part.

There is little time now for taking questions or working the rope line and kissing babies, but somehow the candidates keep it up, pushing themselves to the limits of human endurance as they seek the nomination in what is in effect a national primary.

On her chartered plane Hillary Clinton, who likes to keep the media at arms' length, worked the airline trolley and served peach cobbler (her favourite dessert) to the travelling press corps. For the 45 war-weary reporters assigned to her campaign it was a moment of welcome relief in a mind-numbing schedule of buses and planes and late-night hotel check-ins in unfamiliar cities. After Super Tuesday, an even bigger plane will be needed to carry her entourage. In the midst of the cross-country chase for votes, more black news about the souring economy confronted the candidates. In response, the ground war they are engaged in was matched with an "air war" of saturation advertising.

Mrs Clinton released a provocative ad called "Freefall", showing a man falling through the atmosphere before his parachute stops him. Barack Obama, meanwhile, was demanding "immediate relief" for struggling families as job losses reached 17,000 and the slowest growth since 2002 was recorded.

Mrs Clinton's punishing schedule sees her in Los Angeles, Arizona and New Mexico seeking to shore up support as Mr Obama gains in the polls. On Monday, she will campaign in Boston before arriving in New York to host a 22-state "town hall meeting" by satellite with Bill and Chelsea Clinton.

She arrived at San Diego State University at 1.30pm yesterday, then she was on to San Jose, 460 miles away. By 8pm, Mrs Clinton was to attend a "low-Dollar Fundraiser" at a San Francisco theatre, a step down from the glitzy events that marked the earlier stages of her campaign.

The race for president no longer involves a lengthy cast of characters, but a shortlist of highly competent politicians, any one of whom it is easy to imagine sitting in the Oval Office this time next year. But the four – John McCain, Mr Obama, Mitt Romney and Mrs Clinton – still need to appeal to wavering voters, the media, and their donors. The former governor of Arkansas Mike Huckabee is also still in the race, but without a real shot at winning his party's nomination.

Senator John McCain, at 71, now the Republican frontrunner, runs what may be the most gruelling campaign of all. His fortitude combined with a direct and engaging style paid off handsomely in New Hampshire where he took his Straight Talk Express bus to more than 100 "town hall" events winning the primary almost one vote at a time.

He is a candidate who does not have enough mobility in his hands to comb his hair – as a result of being tortured over five years in a North Vietnamese prison – yet on the stump he is unfailingly patient with his questioners, if occasionally acerbic and mean spirited.

Yesterday, he was at an evening rally in Chicago where a snow blizzard shut down the airport for hours. Then he was due in St Louis, Missouri, a key state in any election, but so rushed was the candidate that he was not even planning to leave the airport after holding a rally in a hangar. At least the McCain campaign now has a decent-sized plane. So hard up was his campaign that many of his staff were either let go or working without pay.

Mr Obama's campaign first took him to New Mexico yesterday, an area with a large Hispanic population, whose support he needs on Tuesday if he is to defeat Mrs Clinton. Then he was off to the snowbound north, with appearances booked in Boise, Idaho, Minneapolis, Minnesota, and St Louis, Missouri, as he kept up his strategy of harvesting votes in the crucial smaller states ahead of next Tuesday.

Senator Obama received a major boost yesterday with the endorsement of California's 650,000 strong service workers union, which had previously backed John Edwards. The SEIU said it wouldn't have time to launch a major field operation on Senator Obama's behalf, but its endorsement is sure to improve his standing with Latino voters, currently favouring Senator Clinton by a hefty margin, and increases the possibility of an endorsement from Mr Edwards himself.

He also received the endorsement of MoveOn.org, the nationwide left-wing grassroots lobby group which has over 1.7m members in the Super Tues states. Members voted to favour Mr Obama over Hillary Clinton by 70.4 to 29.6 per cent.

Coming out of his strong debate performance on Thursday, Mr Obama is on a roll with a war chest of $32m (£16m) from donors to whom he has returned again and again to ask for $25 and $50 a time. Mrs Clinton's campaign does not have that reach and many of her donors have already hit the $4,600 limit that they are allowed to give.