California's governor has taken a lead in banning assault weapons
President Bush is opposing his traditional pro-gun allies over a law banning the manufacture and import of new semi-automatic assault weapons.
Private individuals in the US have been banned for the past nine years from possessing such guns.
President Bush has decided to back moves to renew the ban.
It was originally enacted in 1994 and is due to expire next year.
Bills to renew the legislation have been introduced in both houses of Congress by Democrats, who argue that such weapons are useful only to criminals.
Often the president will agree, of course, with the National Rifle Association - on this issue, he does not
White House spokesman
The National Rifle Association (NRA) - the powerful pro-gun interest group - has been fighting hard to oppose the renewal.
But in a surprise move, the Bush administration - who on a number of issues has supported the NRA - came out in support of the move.
"The assault weapon ban was crafted with the thought that it would deter crime," White House spokesman Ari Fleischer said on Thursday.
"Often the president will agree, of course, with the National Rifle Association," Mr Fleischer added. "On this issue, he does not."
BBC Washington correspondent Justin Webb says the White House, with its eye on the presidential election next year, appears to have taken a decision based on the need to attract wavering voters in the political centre ground - even if it offends core supporters.
Charles Schumer, a Democrat who is co-sponsoring the bill in the Senate, said the NRA had been proved wrong.
"They told you your hunting rifle that your Uncle Willie gave you when you were 14-years-old would be confiscated by some jack-booted thugs who would knock on your door and take away the weapons," he said.
"Today we dare a single American to say that their weapons have been confiscated."