Decision looms on Murkowski amendment to block EPA rules

Murkowski is also weighing use of a blunter instrument: a "resolution of disapproval" that would overturn EPA's "endangerment finding" that greenhouse gases threaten human welfare. The finding is a legal precursor to any future emissions rules.

That plan would also stop EPA's less controversial auto emissions rule, which the agency is crafting as part of a joint emissions and mileage standards policy with the Transportation Department. The resolution could not be filibustered, although it would face massive hurdles even if it cleared the Senate.

Murkowski spokesman Robert Dillon said Monday morning that Murkowski will do something Wednesday to reveal which direction she's heading — either introduce her amendment or take to the floor to tout the resolution of disapproval. The resolution would not be part of the debt ceiling debate.

"Come Wednesday we will all know what she decides," he said, adding that Murkowski is still weighing support for the various approaches.

Dillon said Murkowski has secured the backing of a Democrat for her efforts to block EPA, but he didn't name names.

Kate Sheppard of Mother Jones, who noted Friday evening that there's a mystery Democratic backer, listed several centrist Democrats as possibilities: Sens. Mary LandrieuMary LandrieuYou want to recall John McCain? Good luck, it will be impossible CNN producer on new O'Keefe video: Voters are 'stupid,' Trump is 'crazy' CNN's Van Jones: O'Keefe Russia 'nothingburger' video 'a hoax' MORE (La.), Ben Nelson (Neb.), Jim Webb (Va.), Byron Dorgan (N.D.) and Blanche Lincoln (Ark.).