Tensions over the direction of a sweeping climate change bill boiled over in a House Democratic leadership meeting Thursday, as Energy and Commerce Chairman Henry Waxman (Calif.) lashed out at Rep. Chris Van Hollen (Md.) for appearing to publicly downgrade the measure’s chances this year.

“This is not helpful,— Waxman told Van Hollen, citing reports that presented the party’s campaign chief as opposing aggressive action on the bill, sources familiar with the meeting said.

Van Hollen responded that while he strongly supports a comprehensive energy bill, he believes House Democrats should move first on health care reform — an easier legislative lift — and see whether a climate change bill makes progress in the Senate before subjecting vulnerable House Democrats to a tough vote on it.

The exchange appeared to clear the air between the lawmakers, leadership sources said, but it did not resolve their conflicting views about how to proceed.

The squabble shows the dicey political calculus of pushing a controversial bill labeled “cap-and-tax— by Republicans with an uncertain future in the Senate, even as Democrats fight among themselves for exemptions and carve-outs for their own districts.

The stakes for the package rise this week as Waxman and Ed Markey (D-Mass.), chairman of the Subcommittee on Energy and Environment, seek to slice through several remaining big-ticket tangles in time to stage the markup that they were forced to postpone last week. Amid intense negotiating among Democrats on the committee, the full panel has been summoned to the White House this week for a meeting with President Barack Obama.

Waxman and Markey’s challenge is not limited to the committee. They also face a push from other top Democrats to include strong new incentives for nuclear power. Majority Leader Steny Hoyer (D-Md.), Majority Whip James Clyburn (D-S.C.), Budget Chairman John Spratt (D-S.C.) and Science and Technology Chairman Bart Gordon (D-Tenn.) are among those pushing for nuclear power carve-outs in the bill.

Clyburn said he made it clear to Waxman in a meeting Tuesday that nuclear power has to be taken into consideration to get his support, noting that 54 percent of his state’s electricity now comes from nuclear power.

“It has to be fair,— Clyburn said. He made that point in the Thursday meeting, and Hoyer aligned himself with the position, a source said.

Hoyer and Spratt have giant nuclear plants in their districts whose owners are applying to add what would be the first new reactors in the country in decades.

Nuclear power would benefit greatly from a cap-and-trade system because the technology does not emit carbon dioxide, but pro-nuclear power lawmakers want a variety of other changes, from financing for nuclear plants to investments in reprocessing facilities to reduce nuclear waste.

And Gordon is pushing for an exemption for new nuclear plants from a requirement that 25 percent of electricity come from renewable, non-nuclear power sources.

Waxman and Markey did not include any special incentives for nuclear power in their bill.

Waxman said nuclear power “is part of the many sources of energy we have— but declined to say whether he would include specific incentives.

The pair, meanwhile, have been negotiating — so far without success — with a number of conservative Democrats led by Rep. Rick Boucher (Va.) demanding carve-outs for various industries in their districts — including owners of coal power plants, steel manufacturers, oil refineries, auto manufacturers and the like.

Satisfying all of their demands, while keeping intact the guts of a bill that will slash emissions and be acceptable to the environmental community, is a tall order.

“That’s what’s holding up the bill right now — everybody has their parochial interests they want addressed,— an aide to a Democrat on the committee said.

Waxman said he remains flexible, but only up to a point.

“I feel strongly about not opening the goals and timetables in the bill,— Waxman said, which includes a 20 percent reduction in emissions by 2020.

Other than Speaker Nancy Pelosi (D-Calif.) and Waxman, other Democratic leaders have been hedging on cap-and-trade. Hoyer has left open the possibility of moving an energy bill without cap-and-trade, Caucus Chairman John Larson (D-Conn.) has suggested the timing will slip behind health care, Van Hollen urged caution and Clyburn has been looking to weaken the renewable electricity requirements given that nuclear power is not included.

Judiciary Chairman John Conyers (D-Mich.) acknowledged that the issue has been discussed over and over again at leadership meetings. “Some have suggested that that’s too large a legislative lift to accomplish with the challenge we have ahead with universal health care, troops in Iraq and Afghanistan, the budget, and dealing with the financial crisis,— Conyers said.

“It is not my view that all those things can happen,— he said, adding that there is “a great deal of tension back and forth— in the meetings.

“There are strongly held opinions and numerous voices on all sides,— he said. “We may have to hold off.—