WASHINGTON - Soon after the Senate opened its debate on health-care legislation last week, John McCain strode into the chamber to spearhead his party's opposition to the bill. He offered the Republicans' first amendment and leveled the party's most politically stinging charge, that its Medicare spending cuts would hurt the elderly.

A day later, McCain took the lead in grilling President Obama's team on its newly minted plan for the Afghanistan war. Why, McCain pressed, had the president set a deadline for withdrawing U.S. troops?

"A withdrawal date only emboldens al-Qaeda and the Taliban," he complained.

He's been down and been out, but the Mac is back.

The Arizona senator still strikes his signature pose as war hero and scourge of special interests, but in other ways McCain is cutting a very different profile than he did before he ran for president in 2008.

Gone is the maverick bridge builder who bucked his party on such high-voltage issues as immigration, climate change, and campaign finance. As the GOP has settled on a strategy of unremitting opposition to the Obama agenda, McCain has been front and center on the attack.

At a time when the marquee names in conservative circles are outsiders like Sarah Palin and Fox News commentator Glenn Beck, McCain has emerged as one of Obama's most formidable opponents, especially on two high-profile issues.

On health care, he led the charge in trying to stir older Americans into opposition. And on the Afghanistan war, he has accused Obama of playing into the Taliban's hands.

Senate Minority Leader Mitch McConnell (R., Ky.), with whom McCain has tangled in the past over campaign-finance legislation, now could not be more effusive in his praise.

"He's been a fabulous team player," McConnell said. "All I can tell you is that, in this Congress and post-campaign era, Sen. McCain has been incredible - on message and effective."

It remains to be seen whether McCain could win over hard-boiled conservatives outside Washington who recoil over his past cooperation with Democrats on immigration and other issues.

But McCain is still regarded as enough of a political asset among independent voters that the National Republican Senatorial Committee recently used his voice on recorded phone calls to swing voters in the home states of conservative Democrats like Sen. Ben Nelson of Nebraska in an effort to pressure them to buck their party on health care.

Perhaps predictably, Democrats prefer a different McCain.

"I've always seen two John McCains, one who has the partisan, angry side, and a nice, cooperative, bipartisan side," said Sen. Barbara Boxer (D., Calif.), who has been working on climate-change legislation that McCain has opposed. "I have not seen the bipartisan side in a long time."

Some Democrats see McCain turning more partisan because of bitterness at his 2008 defeat, but his friends say the increasingly polarized political environment makes it harder for anyone to cross party lines.

"He seems to be more aggressive," said Sen. Susan Collins (R., Maine). "The Democrats are finding it harder to reach out to him because the person who beat him to the White House is pursuing a very liberal agenda."

One thing is clear: McCain has not returned from the campaign trail with the sullen, grim attitude that some would-be presidents have had, even though he had to make the same transition after his failed 2000 bid for the GOP presidential nomination.

"I spent a little time dwelling on it but found in 2000, the best thing is to get busy," McCain said in a recent interview. "The best cure for all this is getting back in the arena."

Indeed, McCain was in an upbeat mood Friday after he finished leading his Republican colleagues in an hour-long series of floor speeches lambasting the Democratic health bill. "I thank my colleagues," McCain said, grinning broadly. "It's been a lot of fun."