Senator John Kerry ratcheted up the fight to pass legislation to combat global warming on Wednesday, unveiling a bill as the Gulf of Mexico oil disaster complicates the measure’s already difficult prospects this year.

Kerry, a Democrat, and Senator Joseph Lieberman, an independent, took the wraps off their bill as two important deadlines loom.

Congressional elections are less than six months away and with Democrats facing losses, June or July could be the last chance for them to pass a climate bill this year, before the political atmosphere gets too overheated.