The US president visited Missouri and Michigan yesterday and will be in Ohio and Pennsylvania today.

Having spent the week at his ranch in Crawford, Texas, Mr Bush has returned armed with two freshly recorded campaign advertisements and a new stump speech.

His aim is to set out a vision of his next term before the Republican convention in New York at the end of August, while delivering implicit jibes at his Democratic challenger, John Kerry.

Presidential election campaigns usually kick off in earnest after the Labour Day weekend in early September, and sitting presidents usually try to stay above the fray until it is absolutely necessary to hit the campaign trail.

But with a number of swing states leaning towards Mr Kerry and a dwindling number of undecided voters, aides say Mr Bush will be out of Washington for at least four out of every five weekdays in the next four weeks.

At the end of last month he had more than $64m to fight with, and has spent little of it, while the Democrats have been dominating the spotlight.

The Republicans have spent the past few months lambasting Mr Kerry for being a "flip-flopper" who cannot make up his mind, but they are now turning to a more positive message.

In his new speech Mr Bush says: "We have turned the corner and we are not turning back", and "when it comes to choosing a president, results matter".

The latter is supposed to be a dig at Mr Kerry's 19-year record in the Senate, which the Republicans have derided as undistinguished. The speech is aimed at independent voters, who now comprise only about 10% of the electorate - the lowest figure on record at this stage in an election.

He will concentrate on domestic issues, such as encouraging flexitime at work and unveiling a successor to the No Child Left Behind education policy that has been criticised for being imposed without funding.

During the third week in August, just before the convention, Mr Bush will turn to foreign policy and his plans for "making America safer and extending peace and liberty".

His stops on the campaign trail reflect how much is at stake. Mr Bush won Missouri in 2000 with 50% of the vote to Al Gore's 47%, and lost Michigan by 47% to 51%.

The latest polls show both states leaning towards Mr Kerry, although by so few votes as to be within the margin of error.

Missouri has lost nearly 40,000 manufacturing jobs since Mr Bush took office, but the picture has improved markedly in the past year, with the labour force as a whole growing by 83,000.

"The economy is not a crippling issue for Bush in Missouri, like the war in Iraq might be," said Martha Kropf, a political science professor at the University of Missouri at Kansas City.

"This is a turnout game, and whoever mobilises their base most effectively is going to win the state."

The economic backdrop in Michigan is tougher for Mr Bush. The state has shed 20,000 jobs since last summer, and 211,000 since Mr Bush took office, more than 145,000 of them in manufacturing.

In Grand Rapids, Michigan, the president needs to shore up his support in a normally supportive part of the state.

Jeff Williams, the vice-president of Public Sector Consultants, a nonpartisan thinktank, said that recent polling showed voters there surprisingly split on the president's handling of the war.

The polls show Ohio leaning towards Mr Bush, while in Pennsylvania he trails Mr Kerry by 10 percentage points.