WASHINGTON — A climate change bill championed by Senator Lieberman could test the alliance between the Connecticut lawmaker and Senator McCain, who is opposing the measure despite his advocacy of one of its core components: a cap-and trade program to reduce greenhouse gas emission.

The bill, which the Senate is debating this week, also poses a challenge for Mr. McCain as he positions himself for the presidential election. The presumptive Republican nominee has used his support of a mandatory limit on carbon emissions to court independent voters and distance himself from the Bush administration.

But he will have to square his support for the cap-and-trade system with his stance on the Senate bill, which he opposes because it lacks incentives for nuclear energy production.

On the other side is Mr. Lieberman, the Democratic vice presidential nominee in 2000 who has emerged as one of Mr. McCain’s top surrogates since endorsing his candidacy late last year. Now an independent senator who caucuses with the Democrats, Mr. Lieberman has worked on the climate change bill for the last year with its chief co-sponsors, Senator Boxer, a Democrat of California, and Senator Warner, a Republican of Virginia.

“We’ve got a fight ahead of us, but it’s a critical one and we’re going to win it,” Mr. Lieberman told reporters at the Capitol yesterday shortly before the bill reached the floor. The legislation passed a procedural vote yesterday to begin debate, but it is considered unlikely to muster the 60 votes needed to break a Republican filibuster and achieve full approval.

The Senate Republican leadership opposes the bill, and President Bush has threatened a veto, saying the measure would drive up energy costs by $6 trillion without doing enough to improve the environment.

Mr. Lieberman acknowledged an administration study showing the bill would lead to an increase in gasoline prices by about two cents a gallon annually, but he noted that paled in comparison to spike Americans have seen at the pump over the last year. “The American people would be thrilled” to see gas price increases of only two cents a gallon a year, he said, adding that the legislation’s investments in alternative fuels would ultimately drive down costs by decreasing America’s demand for oil.

Mr. Lieberman said he was confident that Mr. McCain would come around on the bill if a nuclear component could be added through amendments.

“I personally believe Senator McCain will support this bill,” he told reporters, noting that the Arizona senator had reiterated his stance on climate change at a closed-door meeting earlier in the day. “He’ll have the final decision, but he’s been a part of this from the very beginning and I expect that at the end of this, with a critical vote, he’ll be with us.”

Messrs. Lieberman and Warner are submitting an amendment that adds incentives for the nuclear power industry. Whether it will be enough to attract Mr. McCain’s support is unclear. His campaign said late yesterday that Mr. McCain would partner with Senator Graham of South Carolina, a close Republican ally, on a separate nuclear amendment and that it had yet to review the Lieberman-Warner measure.

The likely Democratic nominee, Senator Obama, has not publicly taken a stand on the legislation, but a campaign aide said last night he would announce his support for the bill this week, even though he favors a more aggressive approach to combating global warming.

Democrats are already gearing up to criticize Mr. McCain’s opposition to the bill, with a party spokesman accusing him of “double talk” on climate change.

“Despite all of his spin about being a different kind of Republican, John McCain has consistently opposed efforts to expand renewable energy in the Senate and is now joining President Bush in opposing the climate change bill,” the spokesman, Damien LaVera, said.