Timothy M. Kaine announced Tuesday that he would run to succeed retiring Sen. James Webb, giving Virginia Democrats a marquee candidate for a race that could help determine control of the U.S. Senate and the political direction of a rapidly evolving state.

“I’m running!” Kaine said on his Twitter feed, adding a link to a video highlighting his record as a Richmond City Council member and mayor, as well as governor of Virginia. Kaine, who stepped down Tuesday as chairman of the Democratic National Committee, followed up with an e-mail to supporters about his decision.

His announcement ends a waiting game that began when Webb (D) said in February that he would not run for a second term. On the Republican side, former senator and governor George Allen and Jamie Radtke, the former chairwoman of the Federation of Virginia Tea Party Patriots, are running.

The commonwealth is one of a handful of competitive states that could decide control of the Senate in 2012, when Democrats must defend 23 seats and Republicans must defend 10.

“Virginia is the kind of race that Democrats need to win in order to hold the majority,” said Nathan Gonzales, political editor of the nonpartisan Rothenberg Political Report.

The fight for Webb’s seat will rage on a battlefield that has changed significantly since Allen and Kaine held office.

The last time Allen won a campaign — his 2000 Senate race — Virginia was less affluent, more ethnically homogenous and more firmly in the Republican camp. In the ensuing decade, the state’s population and economic might have increasingly shifted to Northern Virginia, which is home to thousands of newcomers.

And the contest will be expensive: Based on recent campaigns, Kaine and Allen, his likeliest Republican opponent, will probably spend more than $20 million combined.

Battle lines drawn

Kaine had previously suggested that he wasn’t interested in running for Senate. He was lobbied by senior Democrats in Virginia and across the country, most conspicuously by President Obama, a friend and political ally.

Obama, who defied recent history by winning Virginia in 2008, is likely to make the state a central front in his reelection effort in 2012, and each man hopes that the other’s presence on the ticket will help him win.

Democrats have done well in recent years in the Northern Virginia suburbs of Washington, and Republicans remain stronger in the central and southern portions of the state. Exurban counties such as Loudoun and Prince William — both of which are growing rapidly — have been the most hotly contested.

Kaine is the only Democrat to officially announce a Senate campaign, with most others in the state saying they would defer to him. The exception is Rep. Robert C. “Bobby” Scott, who has said he would decide on a Senate bid by July.

Because Kaine’s decision has been widely expected — especially since he told a University of Richmond law school class last month that he was “increasingly likely” to run, according to a spokesman — the rhetorical battle lines for the contest have already been drawn.

Republicans will redouble their efforts to paint Kaine as a liberal and an Obama clone, seeking to separate Kaine from the independent, centrist image he sought to cultivate when he was governor.

“Over the last several years, Tim Kaine has been the most vocal cheerleader in Washington for the reckless fiscal policies and massive expansion of government that have been the hallmark of the Obama administration,” said National Republican Senatorial Committee spokesman Brian Walsh.

Kaine made clear Tuesday that his campaign will be premised partly on the idea that he can replicate the success he enjoyed the last time he held statewide office.

“As governor, I helped lead Virginia through the toughest economy in seventy years,” Kaine wrote in the e-mail. “We cut billions from the state budget, tightened our belt, and made government more efficient.”

Allen campaign spokeswoman Katie Wright said Kaine’s announcement “sets up a clear contrast” between the two likely nominees.

Allen “stands for responsible, smarter government and helping families keep more of what they earn. Allen made job creation his number one priority, resulting in a record number of private-sector jobs and investment in Virginia,” she said.

Kaine’s tenure as governor, Wright said, “was marked by his proposals calling for staggering tax increases and by substantial job losses for Virginians.”

Kaine adviser Mo Elleithee responded that it was “unfortunate that George Allen has resorted to name calling so early in this campaign. Virginians deserve a senator who will focus on our nation’s economic challenges and will bring more civility to Washington.”

As Republicans seek to tie Kaine to Obama and deride his record as governor, Democrats are expected to attack Allen with similar ferocity. Their argument: that Allen was an unremarkable senator in his previous stint and that he brings no particular vision for governance to the 2012 race.

For her part, Radtke suggested Tuesday that neither Kaine nor Allen was the right choice for the Senate. “If we want to have true financial change, we can’t have career politicians,” she said. “They engineered the mess.”

The Allen factor

Although Allen is considered the heavy favorite on the Republican side, he first must overcome an aggressive challenge from Radtke, Hampton Roads lawyer David McCormick and Chesapeake Bishop E.W. Jackson, a tea party activist who filed papers to enter the race Tuesday. Other potential GOP candidates include Del. Robert G. Marshall (Prince William), Prince William Board of County Supervisors Chairman Corey A. Stewart, businessman Bert Mizusawa and wealthy television production company owner Timothy E. Donner.

Sen. Patty Murray (Wash.), chairwoman of the Democratic Senatorial Campaign Committee, said: “Republicans have a real tea party primary on their hands, and George Allen has a long record of spending and debt that will be a major issue in this race. Rank-and-file conservatives are not as enthusiastic about his candidacy as the establishment in Washington, D.C.”

Although Virginia Democrats suffered at the polls last year, with three congressional incumbents losing, party officials say they think that turnout will be significantly higher in 2012 because of the presidential contest.

During the race, Allen must grapple with the legacy of an episode that helped sink his 2006 campaign against Webb — his public reference to an Indian-American campaign volunteer for Webb as “macaca,” a word that is used as an ethnic slur in some cultures.

Kaine, meanwhile, left the governor’s office in January 2010 just as the state’s budget problems were becoming clear and before Democrats took an electoral drubbing fueled by a conservative movement strongly opposed to Obama’s signature policies on health care and climate change.

Democrats, Gonzales predicted, would be “much more comfortable with the race as a competition between their records rather than a referendum on Obama.”

Staff writer Anita Kumar contributed to this report.