His sisters have arrived in Washington for two days of meetings with President George W Bush, coinciding with the March 17th St Partick’s Day celebrations.

The family alleges the IRA was involved in Mr McCartney’s stabbing death outside a Belfast pub in January, and accuses Sinn Fein of a cover-up.

“We hope to get support for our campaign from President Bush and Irish America,” Robert’s sister Claire McCartney said.

“We want all those who played a part in the murder of Robert to be put through the courts,” she said.

According to reports, another sister Catherine McCartney wants to run for political office against Sinn Fein in south Belfast.

Ms Cartney claimed the family wanted to dispel any “romantic vision” Americans may have of the IRA struggle.

“We are now dealing with criminal gangs who are using the cloak of romanticism around the IRA to murder people on the streets and walk away from it,” she said.

She added: “We are going to bring that reality home to Americans who have political and financial influence in Ireland.”

Their high profile campaign has triggered a change in Sinn Fein tactics, which until now has taken a conciliatory approach.

But now the party is warning the sisters to stay out of politics.

“The McCartneys need to be very careful,” deputy leader Martin McGuinness told BBC radio in Belfast.

“To step over that line, which is a very important line, into the world of party politics, can do a huge disservice to their campaign.”

But international support for the McCartneys is mounting.

The family and the Irish Prime Minister Bertie Ahern are to be special guests at a St Patrick’s Day reception to be held at the White House.

Their reception is markedly different to that offered to Northern Ireland’s main republican party.

Sinn Fein leader Gerry Adams hasn’t been invited to the White House, and has been banned from fundraising during his visit to the US.

And in what’s being seen as a deliberate snub, high profile Democrat Senator Edward Kennedy has refused to meet him.