UPDATE: 5:30 p.m. -- The Senate has just defeated all of the EPA-related amendments, including the McConnell-Inhofe amendment which came up 10 votes short of passage. The EPA remains free to regulate greenhouse gases via executive authority.

A Republican proposal to strip the Environmental Protection Agency of its authority to regulate greenhouse gases is slated to come to a vote in the Senate Wednesday afternoon. But Democrats hope to deprive GOP leadership of the 60 votes needed for passage by siphoning off support to several more lenient alternatives.

If successful, the proposal would prohibit the EPA from regulating greenhouse gases in cases where the agency's intent is to address climate change concerns. Supporters of the amendment, sponsored by Senate Minority Leader Mitch McConnell and written by Sen. James Inhofe, R-Okla., ranking member of the Environment and Public Works Committee, say limiting the EPA's regulation authority would boost the economy and help create private sector jobs.

Staff for Sen. McConnell has indicated they do not expect the measure to garner enough votes to pass. But West Virginia Senator Joe Manchin, the lone Democratic co-sponsor of the McConnell-Inhofe amendment, recently told home state radio station WAJR that the measure could attract more Democratic support than Republicans expect. "I just believe that the EPA has totally overstepped its boundaries," he said. "Other people might not, but I believe there will be 13 to 15 Democrats who will vote for it."

The Obama Administration has threatened to slap down a similar measure from the House side if it reaches the president's desk, calling the EPA's authority under 2009's Clean Air Act "common sense." "If the President is presented with this legislation...his senior advisors would recommend that he veto the bill," the Office of Management and Budget said in a statement Tuesday.

In the Senate, Democratic leadership has spent weeks trying to field alternatives to the Republican proposal. And they hope to draw votes away from the measure, which will be introduced as an amendment to a small business bill, by attracting support to amendments with a more lenient approach to the EPA.

One proposal that is expected to garner as many as sixty votes is sponsored by Manchin's fellow Democrat and West Virginia Senator Jay Rockefeller. The amendment would impose a two-year delay on EPA climate regulations on industry - an alternative that Rockefeller sees as more realistic than McConnell-Inhofe. "They want the total elimination of the EPA's role, with no other structure in place," he said in a floor speech last week. "Having nothing in place is irresponsible, unrealistic, and immature."

Rockefeller added that the measure is a "theological" attempt by Republicans to fire up their base for the next election. "It has absolutely no chance whatsoever of becoming law - none. Mine does. Theirs does not," he said.

Another measure that could garner enough votes to pass is sponsored by Montana Senator Max Baucus, and tailored to his Big Sky Country constituency by exempting ranchers and farmers from EPA climate regulations.

But in a letter to the Senate two weeks ago, the American Farm Bureau Federation argued against the amendment. "The EPA plan would inevitably raise fertilizer and energy costs," the letter read. "EPA's approach to regulating greenhouse gases has never been approved by Congress and would set in place a regulatory scheme that the EPA administrator herself has testified would have little to no impact on global climate, absent an international agreement."

A third amendment, put forth by Michigan Senator Debbie Stabenow, would also cater to a home state constituency by exempting the auto industry. Amendments to the small business bill are slated to hit the Senate floor at 4p.m. EST Wednesday.

FOX News' Senior Capitol Hill Producer Trish Turner contributed to this report.