Denver, Colorado - The scramble for the White House was dead even between Democrat Barack Obama and Republican John McCain, according to two separate polls released late Sunday, the eve of the opening of the Democratic nominating convention.

The Gallup Poll showed each candidate with 45 per cent support from registered voters, while a CNN/Opinion Research Corp poll showed a 47-per-cent stalemate.

The polls delivered sobering prospects to Senator Obama, 47, as his party's delegates head to Denver, Colorado to formally nominate him and launch the final push to reclaim the White House from the Republicans.

They were the first national polls taken after Obama named Senator Joe Biden, a top voice on foreign policy in the Democratic Party, as his presumptive vice presidential candidate on Saturday. Normally, presidential hopefuls get a boost in the polls when they name their running mate.

Obama had edged ahead of McCain most of the summer, but his lead has narrowed since early August - to just 1.6 per cent late last week.

Pollsters raised the possibility that Obama has lost ground with women voters disappointed by his vice presidential announcement.

In June, 75 per cent of the backers of his chief rival in the primary campaign - Hillary Clinton - said they'd back Obama, according to CNN figures. That number has fallen to
66 per cent, CNN reported.

Gallup figures however showed that Obama still has 49 per cent of women polled in his corner, versus 39 per cent for McCain. McCain has more support from men - 48 per cent - than Obama, with 42 per cent.

During the four-day convention in Denver, in the foothills of the Rocky Mountains, the party's 4,400 delegates will cast their votes for Obama and Clinton - who won more than 40 per cent of the primary vote - on Wednesday night.

Clinton, who has been campaigning for Obama this summer, is expected to release her delegates to support Obama.

Hundreds of demonstrators marched through downtown Denver on Sunday, under the watchful presence of dozens of mounted police officers and ground forces in full riot gear. They were protesting the war in Iraq and the inequities of the US two-party system.

Security officials were expected to release Sunday's arrest totals later Monday.

Obama is campaigning through the Midwest while his party meets in Denver, and won't appear until Thursday to make his acceptance speech before 75,000 people at Invesco Field.

On Monday, Michelle Obama, the candidate's wife, and other family members, and Senator Edward Kennedy, speaking via videotape, are expected to present the personal side of Obama, still a relative unknown for many Americans.

McCain will head to St Paul, Minnesota, for the September 1 opening of the Republican Party nominating convention.

General elections are November 4. (dpa)