Mitt Romney sought to win over the arch-conservative membership of the National Rifle Association by promising to be the protector of American freedoms and accusing President Obama of launching a stealth attack on the constitutional right to bear arms.

Addressing the NRA's annual convention in St Louis, Missouri, Romney tried to assuage doubts about his commitment to America's love affair with the gun by peppering his speech with references to the US constitution and liberal use of the word "freedom".

He also adopted the NRA's own language by warning that were Obama granted a second term in the White House he would unleash an assault on gun ownership rights in the US. That is precisely the NRA's lobbying argument, despite the fact that Obama has steered almost entirely clear of the gun issue in the past three years to the disappointment of gun control advocates.

"In his second term, [Obama] would be unrestrained by the terms of re-election," Romney said. "Who will stand up for the rights of hunters and sportsmen, and those who act to protect their families? President Obama has not. I will."

Romney paid lip service to the fact that Obama has proved to be non-interventionist on the gun issue, but that did not prevent him from accusing the White House of planning a secret conspiracy. "The right to bear arms is so plainly stated, so unambiguous, that liberals have a hard time challenging it directly. Instead, they've been employing every imaginable ruse and ploy to restrict it," he said.

The NRA's annual meeting kicked off Friday with a display of high-powered assault weapons more customarily seen in Afghanistan or Iraq than in downtown St Louis. The event, expected to be attended by 70,000 NRA members, attracted a slew of top Republicans keen to solidify ties with this core conservative following.

In the run-up to the event the NRA has been filling its warchests for an election campaign in which it expects to spend up to $30m opposing the president's re-election. It has launched a nationwide campaign to sway the presidential election called Trigger the Vote.

It has also been ramping up its rhetoric against Obama, who it depicts as anti-freedom and anti-gun rights. Wayne LaPierre, the NRA's executive vice-president, has been leading the verbal assault with ever more shrill language.

In an article in the NRA's magazine, America's 1st Freedom, he writes that "this year's election could prove the most disastrous in the history of this country". He warns that Obama is dedicated to dismantling the second amendment – the right to keep and bear arms.

Contrary to LaPierre's dire forebodings, Obama has barely spoken on the subject of guns since he entered the White House, and gun controls have been rolled back during his term by seminal US supreme court rulings.

LaPierre gets around that inconvenient fact by warning voters that the president is poised to attack gun rights should he win a second term – an echo of what the NRA said ahead of his first election victory in 2008.

"Obama has spent his entire political career engaged in a stealthy assault on your right to keep and bear arms. [If he wins] re-election to the White House, [he] will be immune from elections and free to misuse his ever-increasing power," LaPierre said.

Other key figures in the NRA have gone even further in their assaults on Obama. Ted Nugent, the ageing rocker who sits on the NRA board, called the president an "anti-American monster in the White House" in comments promoted on the NRA website.

The increasingly hysterical tone of the organisation's pronouncements is only to be expected in an election year, observers of the NRA say. "For the guys running the NRA, every election has to be Armageddon. This year, it's 'Obama is one election away from repealing the second amendment and stealing your guns' – even though he's done nothing but expand gun rights," said Mark Glaze, director of the bipartisan coalition of more than 650 US mayors, Mayors Against Illegal Guns.

The coalition, led by New York's mayor Michael Bloomberg, this week launched a campaign against the NRA's promotion of stand-your-ground laws in the wake of the Trayvon Martin killing.

Glaze sees the NRA's tactics as part of its economic plan: "You can't raise $240m a year without a business model based on paranoia and conflict."

Josh Sugarman, director of the Violence Policy Center, said that the NRA was twisting the reality. "They are taking Obama's lack of action on the gun issue as proof that his second term will be the complete opposite. By saying that they are hoping to drive their people to vote, and to gen up gun sales."

In the immediate aftermath of Obama's election victory in November 2008 there was a spike in gun sales following NRA doom-mongering. A similar phenomenon is being witnessed again in the build-up to the November presidential election.

One online ammunition dealer,, recently called Obama the "greatest gun salesman in America".

The gun manufacturer Sturm Ruger announced last month that its sales were at record levels amid the fears of an Obama second term and that it had put new orders on hold while it cleared the backlog.

The election-year spike belies the general trend in gun sales, which is one of gentle decline. Analysts of the gun industry say that fewer Americans are owning weapons today, but those who do own do so in increasing numbers, while the proportion of high-powered and assault rifles is also growing.

That shift towards military-grade weaponry was on display at the NRA convention in St Louis, where manufacturers were proferring guns normally seen on the battlefield. Nemo were among the retailers offering their wares, which include military-grade assault rifles such as the Omen.

The Montana-based company has a mission statement in which it commits itself to developing weapons that will enable special operations forces "to carry out their missions knowing they are armed with the finest primary weopons available in the world".

Also on display are Glock 9mm hand pistols as used by Jared Lee Loughner in the Tucson mass shooting of US congress member Gabrielle Giffords.

While the NRA is focusing this year on the race for the White House, increasingly its efforts are concentrated at state level where it has found it has the most success in pushing new laws favouring gun makers and owners. It has been an aggressive advocate of the right to carry concealed guns, which has spread across the America, and has fiercely resisted any attempt to regulate assault rifles.

The shooting of Trayvon Martin, and the huge uproar around stand-your-ground laws, particularly among African Americans, that ensued, will be the wild card of the gun debate in this election cycle. "How will the NRA defend these laws that it promoted so heavily?" asked Sugarman. He added that the NRA had remained notably silent on the subject. © Guardian News and Media 2012

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