Sinn Féin will find themselves isolated in the Parliament next week when 627 of the 723 members are expected to condemn the brutal murder.
All the political groups, with the exception of the one to which the two Sinn Féin MEPs belong, have signed up to a strongly-worded resolution.
The six sisters and the partner of Mr McCartney, who was murdered outside Magennis’ pub in Belfast last January, will be in the Parliament in Strasbourg for the debate on Monday.
As well as condemning Sinn Féin and the IRA, the resolution calls on the European Union to fund any civil action the McCartney family might take in their pursuit of justice.
A spokesperson for the GUE group, to which Sinn Féin MEPs Mary Lou McDonald and Báirbre de Brún belong, said they will not support the motion.
They wanted a distinction made between a criminal murder, which they say this was, and a political killing.
“We intended to sign this resolution but it was impossible because it is being used to make a political statement against Sinn Féin and the IRA,” the spokesperson said.