The Republican victories in the New Jersey and Virginia gubernatorial elections have revealed significant cracks in the coalition that swept Barack Obama into the White House last year, and delivered an ominous warning to the President and Democrats: they now own America's ailing economy.

Off-year elections such as yesterday's are notoriously unreliable markers in predicting the political future, but the results strongly suggest that the very voters who were crucial to Mr Obama's victory last year - independents and moderates - are increasingly disaffected with his policies.

The movement that swept Mr Obama to power already looks a diminished force, and a year after his historic win it is striking how the political landscape has shifted, with stubbornly high unemployment and a massive budget deficit making it difficult for the President to turn his 2008 victory into legislative success.

Perhaps the most significant factor in Mr Obama's presidential triumph was the economic crisis, an issue on which he campaigned relentlessly. The most alarming demographic change for Democrats yesterday was that the Republican candidates campaigned on the economy and jobs - and it was those issues that handed them victory.

In exit polls, an overwhelming majority of voters said that they were concerned most about the economy and unemployment. Last November, in Virginia, Mr Obama carried voters worried about the economy by 59 to 40 per cent, and by 61 to 38 per cent in New Jersey. On Tuesday, Bob McDonnell, the Republican candidate in Virginia, was backed by 75 per cent of those votes; in New Jersey, Chris Christie, the Republican, won three in five.

Independent voters - who surged behind the Democrats and Mr Obama in 2008 - moved decisively behind the Republican candidates. That will worry moderate Democrats up for re-election in next year's mid-terms elections, and could make them more reluctant to back Mr Obama's costly reform agenda, such as his proposals on healthcare.

Mr Obama was also swept to power by a huge turnout in young and first-time voters, and African Americans. They largely stayed at home on Tuesday, suggesting that without Mr Obama on the ballot - and with the excitement surrounding his candidacy having now largely abated - they cannot be relied upon to support Democrats next year.

The results will also energise Republicans after a dismal electoral showing in the past four years, helping the party to raise money and recruit candidates.

Yet there were warning signs for Republicans, too. A defeat for the conservative candidate in a congressional by-election in upstate New York - after a conservative rebellion had ousted the moderate Republican candidate - suggests that the ideological civil war inside the party is far from over.

Both Mr McDonnell and Mr Christie played down their socially conservative views and ran as pragmatic fiscal centrists. Their victories suggest that rabidly conservative candidates could scare off independents, the swing voters so crucial to electoral success.