US President Barack Obama and his congressional allies Wednesday insisted they would move ahead on fighting climate change despite a toughening political climate.

Obama appealed for action in his first State of the Union address to the two chambers of Congress -- the House, which approved landmark climate legislation last year, and the Senate, where a similar bill has been delayed.

Obama urged the Senate to "advance" work on climate change, while acknowledging deep disagreements with many Republicans.

"Even if you doubt the evidence, providing incentives for energy efficiency and clean energy are the right thing to do for our future," Obama said to a mixed reception from lawmakers.

"The nation that leads the clean energy economy will be the nation that leads the global economy. And America must be that nation," he said.

The House last year narrowly approved a bill to set up the first-ever nationwide US "cap-and-trade" system, which would restrict carbon emissions blamed for global warming and offer an economic incentive by letting companies trade credits.

Obama did not use the term "cap-and-trade" and voiced support for nuclear power and offshore drilling for oil and gas.

Such ideas are unpopular with many environmentalists but were proposed as part of a compromise Senate package last year that brought aboard one prominent Republican, Lindsey Graham.

But the Democrats last week lost one vote in the Senate when Scott Brown, who has criticized cap-and-trade legislation, pulled off an upset to win the seat of late Democratic icon Ted Kennedy in Massachusetts.

John Kerry, the other senator from Massachusetts and a key force behind climate change legislation, hailed Obama's speech as a sign of continued momentum and vowed: "We can surprise a lot of people."

"The inside-the-Beltway conventional wisdom that this issue has stalled is dead wrong," Kerry said.

Kerry and Obama argue that investment to fight climate change will fuel a new green economy, bringing much-needed jobs.

"Comprehensive legislation will not only speed economic recovery but will also put our country on the path to sustainable long-term economic growth," Kerry said.

But Virginia Governor Bob McDonnell, who delivered the Republicans' response, sharply criticized Obama's climate policies and accused him of "seeking to impose job-killing cap-and-trade energy taxes."

Strobe Talbott, president of the Brookings Institution and a senior official under Democratic president Bill Clinton, said that prospects for a bill were hurt by general opposition to Obama's agenda.

"My sense is that the White House and other advocates of action on climate haven't given up. It's just that the degree of difficulty has gotten more severe," he said.

Ben Lieberman, an analyst at the conservative Heritage Foundation, said that some observers had expected Obama to skirt around any mention of climate legislation.

"He did mention it. But how much this moves the needle -- I think there is still a lot of reluctance in the Senate to move forward with climate legislation," he said.

"I don't see the State of the Union address changing that, but he has made it clear that this is still a priority with him."

The address comes days before a deadline set by last month's Copenhagen summit for nations to submit outlines of how they will fight climate change -- part of efforts to draft a successor to the Kyoto Protocol whose obligations run out in 2012.

The Copenhagen summit saw sharp disagreements, with developing nations pushing for historic polluters such as the United States to do more on climate change.

Daniel J. Weiss, a senior fellow at the left-leaning Center for American Progress, said it was less important for the Senate to move on a cap-and-trade system than to take any action that leads to emission reductions.

"The most important thing is that it meets the 17 percent reduction (by 2020 off 2005 levels) that president Obama talked about in Copenhagen," he said.