Election officials are braced for the biggest election turnout in US history tomorrow as voters finally deliver their verdict on Barack Obama and John McCain to bring to an end a gripping, two-year long campaign.

The excitement generated by Obama's candidacy is expected to see between 130m to 140m Americans vote, easily eclipsing the 121m in 2004.

Although election officials expressed confidence that polling booths across the country will cope, campaigners and political analysts expressed fears that the strain could see long queues and polling stations having to extend their opening hours into the night. The knock-on effect would be results in key states delayed.

With all the major polls putting Obama well ahead, political analysts from both right and left said they expected him to easily reach the 270 of 538 electoral votes needed to win the presidency and many predicted a landslide, taking 350 or more electoral seats.

The Washington-based Pew organisation, one of the most thorough and respected pollsters who accurately predicted the vote in 2004, today put Obama on 52% and McCain on 46%. RealClearPolitics, a website that averages out the main polls, put Obama on 51% to McCain's 44%.

If the polls are borne out tomorrow, Obama would become the first Democrat since Jimmy Carter in 1976 to win 50% or more of the popular vote.

Obama and McCain both began multi-state tours today in Florida, before heading towards their homes states today for what will be a victory party for one and a wake for the other.

Obama told a rally in Jacksonville, Florida: "After decades of broken politics in Washington, eight years of failed policies from George Bush and 21 months of a campaign that has taken us from the rocky coast of Maine to the sunshine of California, we are one day away from change in America."

Disciplined enough to largely hide the relief that began permeating his campaign weeks ago, Obama was unable to avoid providing occasional glimpses throughout the day that he expected to become the 44th president.

In an courteous gesture he might not have made if the race was closer, Obama said he wanted to "congratulate McCain on the great race that he has fought".

Exuding confidence, he told a rally last night: "The last couple of days, I've been just feeling good."

As polling day closes in, he has inevitably become more reflective, telling CBS in an interview this morning that he had found the toughest part of the two-year campaign right-wing attacks on his wife, Michelle. He described this as "just completely out of bounds".

McCain, at a rally in Tampa, Florida, boldly predicted he could still pull out a surprise win. "The pundits may not know it and the Democrats may not know it, but the Mac is back. We're going to win this election," he said.

The Democrats, as well as political analysts, were less sanguine than election officials today about trouble-free voting, saying that with 300,000 precincts across the country, there would inevitably be problems.

Democratic officials complain that in some states Republican election officials are not opening enough polling booths in areas that are predominantly African-American in the hope that some voters, put off by the prospect of long queues, will go home without voting.

Professor Michael McDonald, a specialist in elections at Virginia's George Mason University, said: "I would not be surprised if we had long lines, potentially keeping polling places open longer on election day."

He anticipated problems in states that had not allowed early voting in significant numbers, such as Virginia, Pennsylvania and Missouri.

The Democrats are outspending the Republicans on advertising by at least two-to-one, and in some states four-to-one. They also estimated to have more staff and volunteers out, with 770 offices nationwide compared with McCain's 370.