The pieces are gradually falling into place for George Allen and Timothy M. Kaine to square off for Virginia’s open Senate seat next year in what would be a rare matchup between former governors and statewide political giants and one of the most-watched contests in the country.

Neither man is assured of his party’s nomination to try to succeed retiring Sen. James Webb (D). Kaine, chairman of the Democratic National Committee, said through a spokesman this week that he was “increasingly likely” to run but had not finalized his decision. Allen, who is trying to reclaim the seat he lost to Webb in 2006, must survive a Republican primary against tea party leader Jamie Radtke and perhaps a handful of other candidates.

But operatives in both parties are preparing for the possibility that Kaine and Allen will face each other. And each side expects a race that will be influenced by the candidate who will be atop the ballot: President Obama.

Republicans have made it clear that they would seek to make Virginia’s Senate race a referendum on Obama’s policies, betting that Kaine’s service at the DNC will serve as a drag on his campaign.

“He has been the loudest cheerleader for the stimulus, for the health-care bill, for all the spending and taxing that has been going on,” said Brian Walsh, a spokesman for the National Republican Senatorial Committee.

As a result, Walsh said, “the rhetorically moderate profile that Tim Kaine tried to craft as governor has been laid bare by his tenure at the DNC the last two years.”

Having won Virginia in 2008 — the first Democrat in 40 years to do so — Obama will again lavish attention and resources on the state in 2012. Democrats expect that the effort will drive their voters to the polls and help propel their Senate candidate to victory.

Rep. Gerald E. Connolly (D-Va.) said that having Obama atop the ticket will be a net positive for Kaine — a benefit that Democrats didn’t have last year, when three of their congressional incumbents lost seats.

“We are going to have almost double the turnout we had last year,” Connolly said. “That’s an entirely different electorate.”

Whichever argument prevails, both parties anticipate a hard-fought contest that will be a magnet for national attention — and cash.

In their 2006 matchup, when Allen, the incumbent, lost to Webb, the two spent more than $20 million combined. Operatives in both parties estimate that Kaine and Allen would easily top $30 million in 2012, and that doesn’t include many millions that would be spent in the state by national party organizations, outside interest groups and the presidential campaigns.

From the right, much of that money would be aimed at painting Kaine as a standard-issue liberal.

Would the charge stick?

In a column in Roll Call on Tuesday, Rothenberg Political Report editor Stuart Rothenberg wrote that “it’s far from clear that [Kaine] is still the political powerhouse that he once was, or that some apparently assume he still is.” After having worked to develop a reputation as a centrist, Rothenberg said, now “Kaine can be easily defined by Republicans as a partisan and easily linked to the president.”

But Pete Brodnitz, a pollster with the Benenson Strategy Group who worked for Kaine’s past campaigns, said that his opponents had tried that tactic before without much success.

“For the Republicans to say Tim Kaine is a liberal isn’t really a new strategy,” Brodnitz said.

Connolly agreed that using Kaine’s DNC service against him wouldn’t be effective.

“I think people are quite discerning,” Connolly said. “They understand that was a job . . . and he was a loyal trooper.”

Beyond Kaine’s support of high-profile national issues such as the health-care overhaul, Republicans would also look for ways to tie the DNC chairman to Obama administration policies that affected Virginia. The National Republican Senatorial Committee has linked Kaine to Obama’s approval of a Pentagon proposal to close the Norfolk-based Joint Forces Command, a move that could cost the Hampton Roads region thousands of jobs.

As for Allen, Democrats think that the Republican was an indifferent senator last time around, more interested in running for president than representing Virginia on Capitol Hill, and that he hasn’t articulated a vision for his candidacy.

“He’s talking a lot about what he’s against, but I’m not sure what he’s for at the end of the day,” Brodnitz said.

Because Kaine and Allen are former governors, it should be easy for their campaigns to contrast their records.

Chris LaCivita, a Republican consultant who worked for Allen’s 2006 campaign, said that when Kaine won campaigns for lieutenant governor and governor, he did not have an extensive statewide record to defend.

Now, LaCivita said, Republicans could highlight Kaine’s four years of “incompetence” as governor, including higher taxes, increased spending and mishandled transportation money.

Democrats plan to level a similar charge.

“While George Allen may be loved by Washington lobbyists, he was a disaster as governor and senator,” said Matt Canter, a spokesman for the Democratic Senatorial Campaign Committee. Allen, he said, “saddled Virginia and the nation with so much spending and debt, even rank-and-file conservatives don’t trust him.”

Although Allen is better known and better funded than Radtke, Democrats hope Allen will be hurt by the GOP primary, in which Radtke and possibly other candidates will seek to paint him as insufficiently conservative.

For many Virginians, Allen’s last moment in the public spotlight was a negative one. He lost in 2006 after an incident in which he referred to an Indian American campaign worker for Webb as “macaca” — a racial epithet used in Tunisia, where Allen’s mother was raised.

It’s unclear whether Democrats will attack Allen over that incident. Regardless, Canter said, “it’s certainly already etched in people’s memory.”