Virginia Gov. Mark R. Warner (D) plans to announce Tuesday that he will not challenge Sen. George Allen (R-Va.) next year, leaving the popular Democrat free to explore a presidential bid, several close associates said Monday.

Warner, who leaves office in January, will announce his decision on his monthly radio show on WTOP, said Virginia Democratic Party Chairman C. Richard Cranwell, a Warner confidant.

"He is not going to run for the Senate," Cranwell said. "He really wants to finish out his term strong. He doesn't want anything to distract from that."

A senior political aide who spoke on condition of anonymity because Warner wants to make his own announcement said: "He is not going to run. He is going to announce it tomorrow."

Warner would not comment, but he has said he will make his plans known very soon. His spokeswoman, Ellen Qualls, also would not comment.

The announcement would rob Virginia of what could have been a blockbuster political confrontation in 2006. Allen, a former governor who is ending his first term in the Senate, remains popular at home and is also considering a run at the presidency in 2008.

Warner's decision to avoid an immediate clash with Allen sets up the possibility that the two might meet on a much larger battleground two years later. Warner is barred by Virginia's constitution from running for a second consecutive term as governor.

A Senate battle with Allen now might have forced Warner to emphasize his more liberal credentials in order to draw sharp contrasts between the two. That could damage Warner's efforts to present himself as a moderate alternative to likely candidate Hillary Rodham Clinton for the Democratic Party's presidential nomination.

Warner "might have won. He might have lost," said Larry Sabato, director of the Center for Politics at the University of Virginia. "But the risks were enormous. It could have killed his 2008 presidential candidacy."

Allen's advisers said they are not ready to believe that Warner will pass up a run for the Senate next year.

"Until filing deadline, we'll be prepared to face a wealthy self-funder with statewide name ID," said Allen campaign manager Jason Miller, referring to Warner. "That's how we're approaching the reelection."

Top Allen aides insisted that Allen would beat Warner in a head-to-head contest. Allen completed a two-week "listening tour" of the state this month, traveling from far southwestern Virginia to the suburbs of Hampton Roads.

Voters need a reason to oust an incumbent, Allen advisers said. Warner supports school standards of learning, welfare reform and parole abolition -- all programs started by Allen when he was governor in the mid-1990s.

"We're focused on the reelection," Miller said. "We know the Democrats will decide on their own who their nominee is going to be. We're not taking anything for granted."

Recent public polls, however, have indicated that Warner is popular with nearly three-quarters of the state's residents and would beat Allen in a two-way race if it were held today. Warner, who lost a Senate bid in 1996 against Sen. John W. Warner (R), believes he could have beaten Allen in 2006, aides close to him said.

Steve Jarding, who ran Warner's gubernatorial campaign in 2001, said Warner is weary and not ready to launch a bitter race. Jarding said the governor is not afraid.

"If Mark Warner wanted to be in the U.S. Senate, he'd be a very, very hard candidate to beat," Jarding said. "Fear would be kind of a silly thought."

Jarding said he expects Warner to use the next year to travel across the country assessing the reception to his ideas.

"It's a meat-grinder business at the presidential level," Jarding said. "If that's what he chooses, it's going to be tough. But I won't underestimate him."

Warner's decision leaves Virginia Democrats with no obvious candidate to take on Allen, who has said he plans to run for reelection before making any formal decision about running for president.

Former lieutenant governor Donald S. Beyer (D), who has been mentioned as a possible Allen challenger, has told associates that he is not interested in taking on the senator. Former congressman L.F. Payne (D), who represented central and southern Virginia, and former Navy secretary James Webb have also been mentioned as possible candidates.

Cranwell said he and other Democratic leaders are talking with several possible candidates who could present Allen with a serious challenge.

"You can rest assured that I will beat the bushes until I find someone to run against Allen," Cranwell said. He said he is not disappointed by Warner's decision because he believes that the governor has a bright future in national politics.

"He's brought to Virginia a bipartisan effort that has put Virginia back on track," he said. "We are in need of that on the national level."

Warner has been coy about his future, declining to say whether he intends to run for president, in part for legal reasons: As soon as he confirms he is running, federal laws require that he establish a formal exploratory committee.

But aides say Warner is doing everything he can to be ready to make that announcement. He has formed a federal leadership PAC, called Forward Together, and this summer hired Monica Dixon, a former top aide to then-Vice President Al Gore. Last week, one of the Democratic Party's most experienced Internet specialists formally signed on.

Jerome Armstrong, who served as a key member of Howard Dean's Internet team in 2004, said in a post on his blog that he will be Warner's Internet director. "Warner's said that he wants to be part of the national Democratic Party dialogue, with Forward Together, and I've been hired to facilitate that over the Internet," he wrote.

Virginia Gov. Mark R. Warner (D) Gov. Mark R. Warner, center, will announce his decision about a Senate seat challenge today on his radio show.