Denver, Colorado - Democratic presidential hopeful Barack Obama and his Republican rival John McCain are head to head in the run for the White House, a new poll said Monday.

According to CNN both candidates can count on 47 per cent voter support, the news channel said a few hours before the Democrats kick off their presidential convention in Denver.

One month ago, Obama was comfortably ahead by 7 per cent.

One likely reason for the Obama-slump was disenchantment among the supporters of former first lady Hillary Clinton, who narrowly lost the nomination against Obama.

Twenty-seven per cent of Clinton's supporters said they intended to vote for McCain in the November presidential poll up from 16 per cent in late June, CNN said.

During the four-day convention, more than 3,400 Democrat delegates will officially nominate their candidate Obama.

Clinton released her delegates, telling them they were free to support whomever they wanted, CNN said.

The phone poll with a sample of 1,023 adults was conducted over the weekend. On Saturday, Obama nominated Senator Joe Biden as his running mate.

The convention will be opened with a speech by Obama's wife, Michelle.

The Republican convention will be held next week in St Paul, Minnesota.