President Bill Clinton signed the ban in 1994 in a wave of outrage after a series of multiple killings in schools and fast food restaurants. Even the late Ronald Reagan, a legendary Republican president, lobbied for the law.

But the powerful gun lobby has had the ban in its sights ever since.

And even though the 1999 Columbine school massacre is fresh in many minds, the ban will end at midnight Monday -- seven weeks from the presidential election -- unless the Republican-dominated Congress has an unlikely change of heart.

When the ban was passed, a provision allowed for it to lapse unless Congress voted an extension. Republican leaders have made it clear they oppose the ban.

"It will expire Monday, and that's that," stated Tom DeLay, the Republican head of the House of Representatives from Texas.

A variety of groups tried to get President George W. Bush to pressure Congress to extend the move.

Backed by a poll by the University of Philadelphia's National Annenberg election survey, which found that 68 percent of Americans support the ban, the Million Moms March group bombarded the White House on Friday with phone calls and petitions.

Police chiefs from across America sought a meeting but said it was told the president has a "scheduling conflict".

International Association of Chiefs of Police president Joseph Polisar said: "This year alone there have been more than a dozen officers killed with assault weapons."

Los Angeles Police Chief William Bratton added: "These are weapons of murder. They're not weapons of hunting or collecting.

"The irony is we'll probably have more of these weapons in the United States than there are in Iraq in the hands of insurgents."

Bush had supported an extension of the ban, but said it was up to Congress to find the time for legislation.

The anti-gun lobby blames Bush, who now spends much of his time campaigning in West Virginia, Ohio, Florida, Michigan, Missouri and Pennsylvania. These are all battleground states in the election where the powerful National Rifle Association (NRA) says that a quarter of its four million members live.

Wayne LaPierre, the NRA executive vice president, reckons the assault weapons issue could be worth several percentage points for or against a candidate for the presidency or Congress on November 2.

The NRA is expected to announce soon after the ban expires whether it will throw its weight behind Bush.

It has already condemned Democratic challenger John Kerry as "the most anti-gun presidential nominee in United States history," even though the Massachusetts senator portrays himself as a hunter who supports the right to bear arms.

But Kerry has accused the president of playing with "the safety of the American people."

"The NRA put the squeeze on George Bush and they're spending tens of millions of dollars to support his campaign. So now, the president is saying with a wink and a smile that he'll extend the assault weapons ban if Congress sends it to him."

The law banned 19 types of assault weapons with military features such as bayonet mounts, collapsible stocks and clips of more than 10 bullets. But the debate over its effectiveness has largely been lost in the election sniping.

The NRA and other lobbyist says there is no proof that the ban has had any impact on the number of gun crimes.

But according to anti-gun groups, even after the ban came into effect some manufacturers just made slight design changes to get around the law.

New statistics released Sunday said violent crime fell 14 percent in 2002 and 2003 from the previous two-year period, Attorney General John Ashcroft said.

Over the last 10 years -- the period in which the assault weapons ban has been in effect -- non-lethal firearm violence dropped by two-thirds, he said.

Seven percent of violent crimes were committed with a firearm in 2003, against 11 percent 1993.

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