After months of on-again, off-again markups, the Senate Energy and Natural Resources Committee appears on the verge of passing an energy bill that makes good on Democrats’ pledge to tilt the energy balance heavily in favor of renewables.

But the regional nature of the energy debate will be on full display when the bill moves to the Senate floor, where the key fights will include a controversial first-time national renewable energy mandate as well as provisions to expand nuclear power, and onshore and offshore drilling of oil and natural gas.

Energy Chairman Jeff Bingaman (D-N.M.) before Memorial Day was able to coax enough support from committee Democrats and a lone Republican — Sen. Sam Brownback (Kan.) — to push through the inclusion of a renewable energy standard of 15 percent by 2021 that is the centerpiece of the bill. Bingaman, a five-term Senator from an oil- and gas-producing state, has been the top Democrat on the panel since 1999 and knows his way around energy issues.

The final parameters of the RES have not been set, but in an indication of the diversity of views on the matter, committee staffers spent the recess sorting through 50 amendments proposed by Senators from both parties. The committee may vote on some of the proposed changes this week.

But the difficulty in satisfying a majority of Senators on the issue was on display in the committee markup last month. Sen. Jeff Sessions (R-Ala.) complained the RES would foist disproportionate costs on the Southeast, while Sen. Bob Bennett (R-Utah) called it an arbitrary mandate.

Even supporters complained about the plan. Sen. Bernie Sanders (I-Vt.) called the 15 percent standard, which Bingaman lowered from 20 percent to win votes, “woefully inadequate— and promised to try to increase it during floor debate.

However, the biggest complaint for critics is the exclusion of nuclear power from meeting the targets. Sen. James Risch (R-Idaho) accused Bingaman’s RES of only “picking at the edges— of domestic energy needs, calling nuclear “the only thing that’s going to generate the electricity we need.—

Republicans are vowing a vigorous floor debate on nuclear power, especially in light of President Barack Obama’s proposal to kill the proposed nuclear waste repository at Yucca Mountain, Nev. Bingaman’s energy bill would create a blue-ribbon commission to study alternative disposal solutions, but GOP Senators want action, not study.

“If there is an issue that has been studied in the United States of America, it is nuclear power,— said Sen. John McCain (R-Ariz.), who offered one of several failed nuclear amendments at the markup, although his only failed by a narrow margin.

Sen. Lisa Murkowski (R-Alaska), the ranking member on the Energy panel, last month said Republicans see an opening for nuclear power, which does not produce greenhouse gases, in the climate change debate. Members of both parties are beginning to recognize that if “we’re really going to put our money where our mouth is when it comes to a reduction in emissions and provide for a reliable and affordable energy source, that we must put greater focus on nuclear energy,— she said.

Another unresolved issue that will likely drag out on the floor is the yet-to-be revealed oil and gas title of the bill. A Bingaman spokesman said the centerpiece of the section will be a “robust— inventory of oil, gas and renewable energy resources in the Outer Continental Shelf. But committee Republicans, as well as some Democrats, have made clear that they would like to see expanded production included in the bill as well.

“If we’re going to be energy secure, we’ve got to do everything,— said Sen. Bob Corker (R-Tenn.), who reminded his colleagues last month that an energy bill that met every Senators’ approval is “going to have to offend every single one of us.—

The timing of the Senate floor debate remains unclear. Majority Leader Harry Reid (D-Nev.) has said only that the Senate will wait to move on energy until after the House passes its bill, which could occur this month.

Reid has also suggested he will follow the House’s lead and combine the energy bill with cap-and-trade climate change legislation. While Bingaman and Murkowski have raised concerns that doing so raises the stakes in getting to 60 votes in the Senate, Reid last month said the House bill will serve as a guide for what’s possible in the Senate.

“We’re going to see what the House can do. If they can do it, we can do it,— he said.