By Greg Giroux, CQ Staff

Virginia Republicans on Saturday nominated James S. Gilmore III, a former governor who made a brief and abortive White House run, to defend the Senate seat of retiring four-term Republican John W. Warner .

Gilmore won the GOP Senate nod by defeating state Rep. Robert Marshall in an unexpectedly close contest, by 5,222.3 votes to 5,156.97 votes, among delegates who attended the state Republican convention in Richmond. Bob Berry, a third lesser-known Republican, took his name off the ballot.

Gilmore will now pivot to a much more difficult campaign against Democrat Mark Warner, Gilmore’s successor as governor who left office in early 2006 to high approval ratings. No other Democrat filed to oppose Warner in the June 10 primary election, so he is the Democratic nominee.

Warner, who is not related to the retiring senator, has a huge lead over Gilmore in early independent polling and in campaign fundraising. As of mid-May, Warner had $5.2 million left to spend, compared to just $205,000 cash-on-hand for Gilmore, who also has higher unfavorability ratings than Warner.

At the moment, CQ Politics rates the Warner-Gilmore contest as “Democrat Favored” and considers the Virginia seat the most likely of the 35 Senate seats on this year’s ballot to change party hands.

Virginia law allows the political parties to choose nominees in primary elections or in conventions, and Republican activists last fall chose the latter format. That was one reason why Rep. Thomas M. Davis III , a moderate Republican from northern Virginia, passed on making a Senate race he was expected to enter. He preferred a primary format, which Davis said was needed to help Republicans build the potent political organization needed to defeat Warner.

Davis’ exit from the Senate race paved the way for Gilmore to win the Republican nomination, though Gilmore faced a vigorous challenge from Marshall, a longtime northern Virginia legislator who criticized Gilmore as insufficiently conservative on abortion and immigration issues and also upbraided the Republican Party for straying from conservative precepts.