No longer the Senate's foremost champion of lost causes, Sen. John McCain (R-Ariz.) has begun winning as many battles as he loses and is setting his sights on an array of issues that threaten heartburn for the White House and both parties in Congress.

The closely divided Senate is tailor-made for an independent political entrepreneur like McCain, and he appears ready to make the most of it. Despite speculation that he might retire when his term ends next year, he says he is "very likely" to run again. He already is mapping out causes -- including reform of Congress itself -- that could take awhile.

"John McCain is going to be a very powerful force in the Senate," said Sen. Chuck Hagel (R-Neb.), a friend who does not always agree with McCain. "He will be chairman of one of the most powerful committees in the Senate and has a national base to work from." Besides, Hagel added with a laugh, "rather than take the mountain goat approach -- just ram it -- he has developed some legislative skills and honed them well."

McCain puts it more simply. "We're going to have a lot of fun," he said in a recent interview.

With prospects dimming for another presidential race by the 66-year-old former Vietnam prisoner of war, McCain appears to have settled on the Senate as the bully pulpit for his causes, which have expanded beyond those that dominated his 2000 bid for the Republican presidential nomination.

He follows in the footsteps of Sen. Edward M. Kennedy (D-Mass.), said James A. Thurber, director of the Center for Congressional and Presidential Studies at American University. "When Kennedy realized he was not going to get the [presidential] nomination, he became quite a fine legislator," Thurber said. McCain, like Kennedy two decades before him, is turning into "a workhorse and a real legislator."

After losing the nomination to George W. Bush, McCain returned to the Senate with what appeared to be a second wind and a lot of new ideas, not all of which thrilled the new Bush administration or its Democratic foes in Congress.

Last year, McCain and his allies achieved a nearly decade-long quest to pass legislation to tighten campaign finance rules. President Bush, who clearly disliked the measure, had little choice but to sign it into law.

Among other bills that McCain helped pass was post-Sept. 11, 2001, aviation security legislation, the creation of an independent commission to investigate the government's failure to prevent the attack and congressional authorization for use of force against Iraq. He pushed patients' rights and other health care legislation that passed the Senate but not the House.

In the process, there were few Democrats with whom he did not team up on something, providing the kind of bipartisanship often essential for success in the Senate. It set many Republicans' teeth on edge. But, for a Democrat, "it's good to be with McCain on most issues," said Sen. Russell D. Feingold (D-Wis.), who joined McCain on the campaign finance bill widely known as McCain-Feingold. "People still think my first name is McCain," Feingold said.

For the new Congress, McCain plans similar moves but on a larger playing field.

With Republicans returned to power in the Senate, he is back as chairman of the Commerce, Science and Transportation Committee, which he headed from 1997 until mid-2001, when a GOP defection gave Democrats control of the Senate. The committee's jurisdiction stretches from barge canals to telecommunications, giving McCain a playing field as broad as any in Congress.

Being chairman is like being a "mosquito at a nudist colony," McCain is fond of saying. "So many targets."

Just to name a few of his priorities for this Congress, he says he plans to use the committee's reach to provide broadband access to rural areas, encourage competition and fight monopolies in the communications industry, and combat sports gambling at colleges.

McCain already has used the post to press for tougher efforts against global warming. He joined Sen. Joseph I. Lieberman (D-Conn.) in proposing mandatory limits on industrial emissions of greenhouse gases, putting him in conflict with administration policy and in league with a Democrat who is seeking Bush's job in 2004.

But many of his major interests, new and old, stretch well beyond his committee's purview.

McCain has expanded his health care agenda to include a prescription drug benefit under Medicare. He may seek a middle ground between Democratic proposals and a narrower initiative favored by the administration, aides said. And, in a new phase of his campaign finance crusade, McCain is seeking to scuttle the Federal Election Committee, which he regards as toothless or worse, and replace it with a more forceful agency. He intends to continue his fight against "pork-barrel" spending, including lawmakers' favorite projects, although he has had little success on that front.

The senator wants to pursue military changes and hopes, somewhere along the line, to take on Congress, too, especially rule changes to make it easier to challenge unauthorized spending.

McCain's proposal on global warming, his criticism of Bush's proposed new round of tax cuts as favoring the rich and several of McCain's other domestic priorities put him at odds with Bush on some key issues -- a continuation of their rocky relationship from 2000 to 2002.

But he supported the administration far more than he opposed it in the last Congress: 91 percent in the first year and 90 percent in the second year, according to a compilation of votes by Congressional Quarterly. He generally supports the president's policies toward Iraq, including war preparations, and endorses efforts to change Social Security and Medicare to introduce more competition from the private sector.

It is his opposition to Bush's tax cuts that particularly infuriates conservatives, who see McCain as wandering too far from his earlier moorings on the right. Stephen Moore, president of the Club for Growth, which advocates tax cuts, is trying to persuade a tax-cutting conservative to challenge McCain in the 2004 GOP primary in Arizona. Moore thinks McCain is a shoo-in in the general election but could have trouble in a conservative-dominated primary.

McCain does not appear to be worried.

John McCain, who had sought the Republican nomination for president in 2000, says he is "very likely" to seek reelection to the Senate.