As the Senate slogs through complex amendments to comprehensive energy legislation Wednesday, it remains unclear whether the backlog of potential changes could be dispensed with in time for the scheduled start to the August recess.

Majority Leader Bill Frist (R-Tenn.) has threatened to keep Members in session past Friday in order to finish the bill, which faces hundreds of amendments.

“We’re staying on this bill,” Frist said Tuesday. “We’ve spent 16 days on it so far. This is the week we bring it to completion.”

“I hope I don’t see slow-walking by Democrats,” he added.

Republicans have criticized Democrats for filing almost 400 amendments, saying their complaint that wrapping up this week stifles debate is just an excuse not to pass a bill.

On Wednesday — which Energy and Natural Resources Chairman Pete Domenici (R-N.M.) termed “environmental day” — the Senate turned to a number of clean air and fuel issues.

Domenici said the Republican leadership was yielding to Democrats by allowing the somewhat unrelated provisions to be debated.

And Democratic Senators seeking their party’s presidential nomination wanted time to talk about environmental issues so they will get it Wednesday, he said.

Already today, the Senate saw a long debate over what steps Congress should take to prevent future manipulation of the electricity market, such as the kind practiced by the now bankrupt Enron Corp.

In a tight vote, the Senate rejected a provision authored by Sen. Maria Cantwell (D-Wash.) to write more safeguards into the measure.

Domenici says his repeal of the Public Utilities Holding Companies Act includes consumer protections.

On Tuesday, the Senate handily defeated a politically sensitive provision that would have raised Corporate Average Fuel Efficiency standards governing automobile gas mileage.

The huge number of amendments has some Republicans, such as Budget Chairman Don Nickles (R-Okla.), questioning whether Democrats want this bill at all.

But Minority Leader Tom Daschle (D-S.D.) said it has been hard to stay on task as Frist has brought other issues to the floor in the middle of the energy debate.

“Well, I would like to think that it’s at least 50-50” that the Senate will finish this week, Daschle said Tuesday. “But I must say, with all that we have to do, I think it would be a very heavy lift. We’re going to do the best we can but we keep being diverted by these judge votes, by perhaps the free trade agreement votes, by other issues. … The supplemental vote could come up,” he added.

Frist dismissed that complaint, saying each cloture vote only consumes 45 minutes.

Wednesday the Senate again rejected ending debate and bringing the nomination of Miguel Estrada — whom Bush has tapped to fill a seat on the U.S. Court of Appeals for the D.C. Circuit — to the floor.

On Tuesday the nomination of Priscilla Owen — named to the 5th Circuit — was also prevented from moving to the floor.

Domenici, meanwhile, urged Midwestern Democrats eager to boost ethanol as a fuel source to get on board with his wide-ranging bill.

Noting that Daschle has already touted his leadership on the ethanol issue, Domenici said this week: “He better get with us and help us move this along or there won’t be an ethanol” provision.

The bill, which stalled last year, is a Bush administration priority and implements many of the president’s recommendations, including increasing domestic oil and natural gas production.

A key feature would build a giant pipeline to move natural gas from Alaska to the lower 48 states.