Address in St Louis comes against backdrop of Trayvon Martin shooting, and the 'stand-your-ground' laws the NRA supports

Mitt Romney had had a long and uneasy relationship with the gun lobby but he will try to overcome that on Friday afternoon when he addresses the annual conference of the National Rifle Association in St Louis in the hope of unlocking tens of millions of dollars for his White House campaign.

His speech will be a difficult balancing act, being delivered against the backdrop of the Trayvon Martin shooting and the 'stand-your-ground' laws which the NRA is championing.

Romney is seeking the financial backing of an organisation that in the past has devoted $20m to $30m in supporting presidential candidates. At the same time, he has to be careful to avoid alienating the large swathe of people uneasy over the stand-your-ground laws in Florida and elsewhere in America in the light of the Martin shooting.

Although Barack Obama has made no significant move against existing gun laws, the NRA does not trust him and is planning to campaign against him in the November election. The NRA leadership has already said it plans to support Romney.

Normally a trip to the NRA is an easy one for a prospective Republican president but Romney's position on guns, like his position on health reform, is ambiguous.

Many of the NRA's 4 million members, interviewed by various media outlets in the run-up to the St Louis conference, have expressed suspicion and distrust over Romney's record on gun laws and are not reassured by his claims that to be a 'hunter'.

Romney so far has had little to say about Trayvon Martin and stand your ground, restricting himself to a short statement describing his death as a tragedy and calling last month for "a thorough investigation" to reassure the public that justice was being done impartially.

Part of the suspicion of Romney can be traced back to his failed campaign for the Senate in 1994 in liberal Massachusetts where he supported the Brady Bill, which places curbs on gun sales and assault weapons. He also boasted at the time of his independence from the NRA. "I don't line up with the NRA," he said.

As governor of Massachusetts, he pledged to do nothing to reverse the state's relatively tough gun laws. The NRA withheld its endorsement of him during his campaign for the governorship.

In the run-up to his 2008 bid for the Republican presidential nomination, Romney said he was a member of the NRA: true, but papers showed he had joined only months before joining the campaign trail. He said he was a gun owner, but the gun in question belonged to one of his son's. He also insisted he was a hunter, though he had only been out twice. He has since made further trips, however.

The NRA says it plans to spend more in advertising and other campaign than previous presidential elections.