U.S. Sen. George Allen formally opened his Republican campaign for re-election Tuesday, but refused to guarantee that he would stay for a full six-year term.

"I'm running for re-election — just that," Allen told about 100 supporters outside a microbiology research center in Prince William County.

Allen is among several Republicans who are actively exploring a presidential bid in 2008.

Allen, 54, is seeking a second Senate term as he travels the nation building the foundation for a presidential race. He has traveled several times to New Hampshire, Iowa and other pivotal primary and caucus states.

Democrats have seized on Allen's obvious White House ambition, portraying him as "bored George," distracted and uninterested in his Senate seat.

Unopposed for renomination, Allen in November meets the winner of an already bitter battle between two Democrats. Harris Miller, a wealthy former Capitol Hill lobbyist for the information technology industry, and James Webb, a military-suspense novelist and former Republican who was secretary of the Navy under President Reagan, will meet in the June 13 Democratic primary.

Neither Democrat has run for elected office before.

Allen has been content to allow the Democrats to savage each other: Miller contending that Webb has discriminated against women and opposes racial affirmative action and diversity programs, while Webb notes Miller's lack of military or national security knowledge in a time of terrorist threats.

Allen is one of the most formidable fund-raisers in Congress, taking in nearly $7 million in campaign donations last year, and has a reputation as a tireless campaigner who relentlessly hammers his opponents.

Allen overcame huge deficits in every statewide poll in 1993 to easily defeat Democrat Mary Sue Terry in his election as governor. During his term, he was praised for reforming welfare and abolishing parole, and panned for slashing the state payroll, open clashes with the U.S. Environmental Protection Agency and reining in the state's environmental regulatory agency.

Nearly three years after his term ended, he unseated a two-term incumbent, Democratic Sen. Chuck Robb, in the 2000 election.

In 2004, with public support for the war in Iraq ebbing, Democrats hoped to win a narrow majority in the Senate. Instead, the GOP expanded its majority by four seats, including that of former Senate Democratic Leader Tom Daschle or South Dakota. Allen, who was finishing two years as chairman of the National Republican Senatorial Committee, got much of the credit for aggressive fund-raising and sometimes ruthless campaign support in those races.

Allen is the namesake son of the Hall of Fame coach of the Washington Redskins and Los Angeles Rams and grew up in a household dominated by the brutal world of pro football.

He's particularly popular in rural Virginia. Allen wears cowboy boots with even the most formal attire, has a sunny, country-boy persona and nestles a pinch of smokeless tobacco beneath his lower lip in unguarded moments. At campaign appearances — outdoors or indoors — the former University of Virginia quarterback will fling a football into the crowd.

But he can be coldly blunt. At the 1995 GOP state convention, he used old-school locker-room rhetoric in exhorting a howling Republican throng to end the Democrats' legislative majority that year and "enjoy knocking their soft teeth down their whiny throats."