Illinois House Speaker Michael Madigan says passing a same-sex marriage bill out of the Illinois House will be “very difficult.”

Mr Madigan told reporters on Wednesday that he backs the measure in allowing same-sex couples to marry.

The Senate approved it last month and it awaits a House floor vote, but lawmakers aren’t sure when they’ll call it.

Mr Madigan says the House is currently 12 votes short.

The Chicago Democrat also says he’s been contacted by Cardinal Francis George of Chicago, who opposes equal marriage.

Advocates for giving same-sex couples the same marital rights as heterosexuals acknowledge there’s more resistance in the House than in the Senate, but they’re confident of House approval soon.

On Valentine’s Day the Illinois Senate, which is controlled by the Democrats, voted 34-21 to approve the bill to legalise same-sex marriage.

Governor Pat Quinn says he’ll sign the bill if it comes to his desk.

Earlier on Wednesday, President Obama, whose home state is Illinois, suggested that the spirit of the US Constitution should take precedent in the equal marriage debate.

The president said he could not imagine a state’s same-sex marriage ban being constitutional.