President Bush has warned Americans against handing over power to the Democrats in November's election.
Democrats say Bush is vulnerable
In a speech to Republican governors, he said his rivals would leave the US "uncertain in the face of danger".
Voters will decide "between two visions of government - one that encourages enterprise and one that raises taxes".
Mr Bush's approval rating has slumped in recent weeks amid fierce attacks from Democratic contenders over the war in Iraq and the economy.
The Democrats have not yet chosen their presidential candidate - but front-runner John Kerry has capitalised on strong anti-Bush feelings among party voters.
The BBC's Katty Kay in Washington says the president would not be lashing out at potential rivals this long before the election if the White House was not worried.
"Our opponents have not offered much in the way of strategies to win the war (on terrorism), or policies to expand our economy," Mr Bush said.
Come November, he added, the voters will have "a very clear choice".
"It's a choice between keeping the tax relief that is moving this country forward - or putting the burden or higher taxes back on the American people.
"It's a choice between an America that leads the world with strength and confidence - or an America that is uncertain in the face of danger."
Mr Bush also said that if re-elected he would "keep our enemies on the run and extend the frontiers of liberty".
Although he defended his policy on Iraq and said it had been a "tough choice", he acknowledged that he had erred on the side of caution given the situation in the world after the 11 September attacks.
But, he added, " we acted, we removed him. And the world is better off for it in my judgement."
Forced to start early
The speech on Monday, given to the Republican Governors' Association, came a week before Mr Bush is due to launch a multi-million dollar advertising campaign.
Democratic front-runner, John Kerry said the Democrats had forced the president to begin his re-election campaign earlier than planned.
President Bush has warned voters against the Democrats
"We now know that George Bush is on the run because he does not have a record to run on," Mr Kerry said in a speech in New York, where he attacked the White House over jobs, the budget deficit, and
Mr Kerry added that President Bush had a credibility gap, because "the truth is his record is very much at odds with his rhetoric".
He went on to say that the Democrats and the America president had a differing vision for America: "We believe in an America that's creating jobs not losing them."
According to several opinion surveys, the president's approval ratings have fallen below 50%.
One recent poll suggested that if the election were held now, he would lose to either to John Kerry, or Senator Kerry's closest rival for the Democratic nomination, John Edwards.