Barack Obama has hit the crucial 50 per cent mark in the polls for the first time after an impressive Democratic convention and concerns over John McCain’s choice of the Alaska governor Sarah Palin as his running mate.

The Democratic nominee now holds an eight-point lead over Mr McCain – 50 per cent to 42 per cent – according to today’s daily tracking poll released by Gallup.

Mr Obama has bounced back from a poll low earlier this month when the two presidential candidates were tied on 44 per cent. Undecided voters have dropped from 11 per cent to 8 per cent, suggesting that those who have recently made up their minds are opting for Mr Obama.

Today, the McCain camp was pinning its hopes on a successful convention to peg back its rival.

Mrs Palin is due to address delegates tonight and Mr McCain’s top advisers have been busy prepping her for a speech that they hope will dispel doubts over her experience, grasp of national issues and the various personal and political controversies that have been swirling around her since the selection was announced on Friday.

"Based on the fact that Obama didn't even see a bounce until Wednesday or Thursday [of his convention last week], and the fact that the Republicans didn't even have a convention last night - for all of those reasons, we will be looking to see whether there will be a bounce for McCain in the next several days," said Frank Newport, Gallup editor-in-chief.

The revelation that Mrs Palin’s 17-year-old daughter is pregnant will have had no impact on today’s figures as polling interviews stopped on Monday just as the news was breaking.

However emerging details of the Wooten scandal – allegations that Mrs Palin abused her office to try and get her former brother-in-law fired as an Alaska state trooper – could have made an impact.

The Gallup results suggest the convention has helped Mr Obama win over both white male voters and former supporters of Hillary Clinton, where he has seen an increase of 5 and 11 points respectively. He has picked up support in the eastern states and in the west but in the south and Midwest has seen a slight loss to Mr McCain.

It also appears that the selection of Mrs Palin has failed to have the intended effect of winning over female independents, with the Democratic nominee extending his support among independents of both genders. Mr Obama also enjoys a 51-45 per cent lead in a new Rasmussen survey.

The scrambled start to the Republican convention – scaled back dramatically due to Hurricane Gustav – may have hit Mr McCain, but perceptions that he has swept aside partisan politics in the face of a national emergency could have contributed to his 1-point increase in support since last week, when the Democratic convention was in full swing.

A breakdown of battleground states by Real Clear Politics shows Mr Obama with healthy leads in Michigan, New Mexico, Iowa, Oregon, Pennsylvania, Wisconsin, Minnesota, New Jersey and Washington, while in New Hampshire and Colorado he maintains a slight edge. Mr McCain leads by upwards of four points in Indiana, Missouri, North Carolina, Montana, Georgia, Texas, Mississippi and Louisiana, with lesser leads in Ohio, Florida and Nevada.

According to the breakdown - which uses averages of polling data from a number of national and local sources - the pair are locked in a tie in Virginia. RCP's electoral college count shows Mr Obama leading Mr McCain by 228 to 185, with 125 votes remaining a toss-up. A candidate needs at least 270 electoral college votes to win in November.

Last night, with the floods subsiding in Louisiana, President Bush began to rekindle Republican fires with a convention address beamed from the White House which heaped praise on Mr McCain and his running mate.

Mrs Palin has been holed up at her suite at the Hilton Minneapolis since Sunday night, as Mr McCain’s top advisers brief her on the nominee’s policy positions, national issues and how to introduce herself to an audience of millions.

The Alaska governor is sure of a rapturous welcome from the Republican convention, where delegates have rallied to her defence following the news that her daughter Bristol is having a baby with her boyfriend-turned-fiance.

As a staunch social conservative and lifetime member of the NRA, her selection sent the party’s powerful social and evangelical conservative base into peals of delight.

But her speech must win over a far tougher crowd – an America that hardly knows her and has been bewildered by a series of dubious revelations – from her past membership of the fringe Alaska Independence Party to an audio recording of her laughing while a cancer-surviving political colleague was called a "bitch", "cancer", and mocked for her weight.

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