Sen. John Warner last week announced his decision to end his 30-year Senate career in 2008, tipping the first domino in what is expected to be a fierce open-seat battle to succeed the Virginia Republican — one that has far-reaching implications for state and national politics.

Warner’s retirement sets up an all-but-certain GOP battle between Rep. Tom Davis and former Virginia Gov. Jim Gilmore. Among Democrats, all eyes are on popular former Gov. Mark Warner, the party’s dream candidate who would be considered the early frontrunner if he enters the race.

A decision from Mark Warner is expected to come within the next week or so, with most insiders believing the former governor will run.

Regardless of what Mark Warner does, Republicans are bracing for a nasty nomination fight.

An ideological bloodletting between Gilmore, an anti-tax conservative, and Davis, a social moderate, seems certain, though other possible Republican contenders have been mentioned. However, the method for how the party will choose its nominee will not be decided until later this fall and the choice could heavily impact the outcome of the nomination fight.

State Republicans can opt for a primary or convention and the 82-member state GOP central committee will meet in October to vote on the method.

A primary would favor Davis, who already had more than $1 million in the bank at the end of June and has a base in vote-rich Northern Virginia. The Congressman and his allies also want a primary because it would help him build name ID across the state.

But a convention, a smaller forum dominated by social conservatives and party activists, would seemingly heavily favor Gilmore. Davis has a legislative record that reflects his moderate social views, including being pro-abortion rights, and Gilmore sympathizers argue it would be almost impossible for Davis to win in a convention setting.

While almost everyone held their partisan fire Friday in deference to Warner, the Club for Growth wasted little time launching the first attack on Davis.

Gilmore’s most prominent accomplishment as governor was the near elimination of the state car tax, and the conservative anti-tax group is seen as likely to back his bid if he runs.

“Tom Davis has one of the most economically liberal records among Republicans in the House,” Club for Growth President Pat Toomey said in a statement. “Since Republican voters in Virginia are decidedly economic conservatives, it’s hard to see how Davis could win a statewide primary.”

Davis operatives argue otherwise.

“This is a guy who hasn’t lost a campaign and knows what’s required to win not only in a primary but in a convention,” said Chris LaCivita, a longtime GOP operative who is working for Davis. “I think it would be premature for people to say that Tom Davis can’t win a convention. … He can be very competitive in a convention.”

LaCivita argued that there are portions of Davis’ voting record that will be attractive to conservatives and also laid out what is likely to be one of the Congressman’s core campaign themes: electability.

“At the end of the day, Tom Davis is the best positioned Republican to not only win a primary but to be competitive in a general election and beat Mark Warner,” he said.

Proponents of a primary argue that the statewide format produces the strongest possible nominee. But it also is a far more costly and nasty endeavor for the eventual winner.

Gilmore would be starting a Senate campaign way behind Davis in the fundraising department. The former governor made a short-lived run for the White House this year, and when he dropped out of the race in July his campaign had more debt than cash on hand.

Virginia Republicans already are slated to hold a nominating convention next May to elect a state party chairman, at which time they also could hold a Senate nominating convention that could draw upward of 12,000 people.

A primary is paid for by the state, while a convention is funded by the party.

Some Republicans estimate that a convention could cost the state GOP several hundred thousand dollars, which would be financed through hard-dollar contributions.

A spokesman for the state party said the choice of a convention or a primary would come down to the will of the central committee members and predicted that cost is unlikely to be a big motivating factor.

“The folks who want a convention will probably be more than willing to foot the bill,” said the spokesman, Shaun Kenney.

Many GOP insiders believe Davis doesn’t have enough support on the state central committee to avoid a convention. The group is dominated by conservatives and only about a third of its membership comes from Northern Virginia.

In announcing his retirement on the campus of the University of Virginia on Friday, Sen. Warner stressed bipartisanship and a willingness to work across party lines.

Even though his views are more in line with an ideological moderate like Davis, Warner said he does not plan to back a successor in the 2008 race.

He said he hoped an experienced person would step up and specifically singled out Members of Congress as an example.

“That’s the type of experience I hope will come forward to win this election,” said Warner, the second-longest serving Senator in Virginia’s history.

Mark Warner challenged John Warner in 1996, garnering 47 percent of the vote.

Mark Warner made a statement praising his one-time foe Friday, but he did not address his own plans or possible interest in seeking the Senate seat.

“On a personal note, I know it is unusual for two people who ran against each other to become friends after the election, but John and I did,” the former governor said. “I’m proud to call John Warner a friend and wish him nothing but the best in the years ahead.”

If Warner opts not to run, former Lt. Gov. Don Beyer is mentioned as a possible Democratic contender. Beyer, a wealthy car dealer from Alexandria, lost a 1997 gubernatorial bid to Gilmore but has re-emerged on the state’s political landscape in recent years and is viewed by many observers to be girding for a return to elected politics.

While many insiders view Mark Warner, who almost launched a 2008 presidential bid and could wind up on the short list of Democratic vice presidential prospects next year, as likely to run for the Senate, the decision has many implications.

If he decides to seek the Senate seat, it would effectively remove his name from VP consideration. Also, in the past Warner has made no secret of his desire to serve as governor again and some observers speculate he may have interest in a 2009 gubernatorial bid.

Meanwhile, Davis’ expected departure gives House Republicans another competitive open seat to defend. The Fairfax and Prince William counties-based 11th district is a pure swing seat that has been trending more and more Democratic in recent years.

Fairfax County Board of Supervisors Chairman Gerald Connolly (D), who faces re-election to his current post in November, is widely expected to run to succeed Davis. Former Rep. Leslie Byrne (D-Va.) is another possibility.

Republicans mentioned include state Sen. Jay O’Brien, Prince William County Board of Supervisors Chairman Corey Stewart, state Del. Tim Hugo and Sean Connaughton, who is now head of the U.S. Maritime Administration.