Former Virginia Gov. Timothy M. Kaine, left, and former Sen. George Allen participate in a July debate. (Steve Helber/AP)

Timothy M. Kaine has jumped ahead of George Allen for the first time in their U.S. Senate race, according to a new Washington Post poll in Virginia, changing the complexion of a nationally watched contest that could help determine which party will control the chamber.

Kaine (D) leads fellow former governor Allen (R) among likely voters by 51 percent to 43 percent, and Kaine is ahead among all registered voters by an identical margin in the hard-fought contest to succeed the retiring James Webb (D).

The survey’s results mark a significant shift: The past two Post polls about the race, taken in May 2011 and May 2012, showed a tie among registered voters, and several more recent surveys have shown a deadlock.

A Quinnipiac University-CBS News-New York Times poll released Wednesday also has Kaine in the lead.

With seven weeks until Election Day, Kaine appears to have a clear edge, helped by a growing lead among women and a significant uptick in support among seniors and residents of the area surrounding Richmond, where he served as mayor. Although Kaine previously lagged behind President Obama, his support now tracks closely with the top of the ticket: Obama leads his Republican challenger, Mitt Rom­ney, in Virginia in the Post poll by the same eight-percentage-point spread among likely voters.

Virginia by no means has the only tightly contested race in the country. Democrats are fighting to keep their Senate majority by holding seats in Montana, Missouri and Wisconsin, while Republicans are battling to keep control of Indiana, Maine and Massachusetts, among others.

But no state is considered more integral to winning the Senate and the White House than Virginia, making the new poll’s shift toward Kaine and reinforcement of Obama’s lead especially welcome signs for Democrats. And no other state features such a high-powered matchup: Allen and Kaine are proven fundraisers with high name recognition. The race is Allen’s fourth statewide campaign and Kaine’s third.

The survey results come as the contest enters a decisive phase. Kaine and Allen are set to debate Thursday in McLean, with two more face-offs scheduled in October. Both candidates have multimillion-dollar ad campaigns inundating the airwaves — Kaine’s team introduced two new spots Wednesday portraying him as a bipartisan dealmaker — and outside money is pouring into Virginia.

Although Kaine’s campaign has consistently outraised Allen’s, outside conservative groups such as Crossroads GPS have pumped millions of dollars into negative ads against the Democrat. Yet voters’ opinions of Kaine have improved significantly since the previous Post poll.

Fifty-four percent of registered voters now have a favorable impression of the Democrat, while 34 percent view him unfavorably. In May, the score was 41 percent to 41 percent. Allen has a margin of 51 percent to 35 percent, similar to the 47 percent to 31 percent he recorded in May. For both men, the percentage of voters expressing no opinion is dwindling quickly.

In Northern Virginia, Kaine has a 13-point lead among registered voters and a 22-point lead in the close-in D.C. suburbs. Allen has a 14-point advantage in central and western Virginia.

But Kaine also is ahead in the Tidewater region, which includes Hampton Roads, and has opened up a 17-point edge in the area that includes Richmond and points east of the city. He trailed there by double digits in May.

Ronald N. Kroll, 71, a retired doctor from Bruington, said he prefers Kaine because of his record.

“We lived in Richmond at that time [Kaine was] mayor, and he did a very good job in very difficult circumstances,” Kroll said, adding that the Democrat was also governor during “tough times financially, and he stuck by the same decisions he did as mayor.”

Susan DiGiovanni, 54, of Midlothian also fondly remembered Kaine’s mayoral days.

“I find him to be a very honest and trustworthy person,” she said. “I believe he is somebody that would fight for Virginia. George Allen, I think, is more for big business and less representative of what I would like to see happen for the country.”

Kaine has a growing advantage among women, who prefer him by 14 points — seven points higher than in May. That trend comes even though Allen spent several weeks this summer airing television ads aimed at improving his standing among women. Kaine has battled to a tie among men, a group that tilted in Allen’s direction in the previous two polls.

Seniors also appear to be gravitating toward Kaine. The two men are essentially tied among voters 65 and older, whereas Allen led by 2 to 1 in May. Democrats have sought to link Allen to the Medicare reform proposals advanced by House Budget Chairman Paul Ryan (Wis.), the Republican vice presidential nominee.

The Post poll asked Virginians whether they preferred to have Medicare continue as a government-run program or become one in which the government gives seniors a fixed amount of money to buy private insurance or Medicare — similar to Ryan’s plan. Fifty-six percent of registered voters said they wanted to keep the program the same, while 35 percent endorsed the reform proposal.

DiGiovanni, a doctor and assistant dean of medical education at Virginia Commonwealth University, said the fight over the federal health-care law factored into her decision.

“I believe that although Obamacare is not perfect, it certainly goes much further than any other president has gone to try to guarantee health care for our citizens,” she said. “I feel like Allen would be with the Republican camp in repealing that and going backward.”

Few Virginians called health care the most important issue in the Senate race. Thirty-nine percent of voters cited the economy or jobs as paramount, and no other single topic exceeded 5 percent.

Virginia voters are more unsettled in their choice for senator than they are for president. Twenty-eight percent of registered voters are undecided or are open to switching their selection, compared with the 19 percent who are up for grabs in the presidential contest. Persuadable voters divide about evenly between Allen and Kaine in the poll, but few have strong impressions of either candidate, leaving plenty of room for debates and nonstop TV ads to fill the gaps.

Doug Lathrop, 41, of Arlington County, a Republican and lobbyist for a life insurance company, is one of many who have not made up their minds in the Senate race.

“I’m pretty sure I would vote for George Allen, but I wouldn’t say categorically I’m leaving out the chance I’d vote for Tim Kaine,” he said.

The fact that both men served as governor, Lathrop added, “causes me to leave the door open. I don’t think the state of Virginia did terribly bad under Governor Kaine, so that’s why I’d give him a look. I think they’re both competent public officials. I don’t think either one has separated himself with the argument that they’d be a better public servant than the other guy.”

Beth B. Lipphardt, a disabled Suffolk resident, said she will vote for Allen as “the lesser of two evils.”

“I have likes and dislikes of both of them,” she said, explaining that although Kaine “was a very good governor,” she dislikes that he served as Democratic National Committee chairman.

Republicans have sought to use Kaine’s DNC service to link him to Obama’s more controversial policies. Equal shares of respondents cited Kaine’s work for the DNC as a reason to support or oppose him.

The poll was conducted by telephone Sept. 12-16 among a random sample of 1,104 Virginia adults, including 934 registered voters and 847 likely voters. Interviews were conducted on conventional and cellular telephones, and carried out in English and Spanish. The margin of error for registered and likely voter samples is plus or minus four percentage points.

Jon Cohen, Peyton M. Craighill, Errin Haines and Laura Vozzella contributed to this report.