WASHINGTON — In a rare move, Mayor Bloomberg will make an endorsement today in an out-of-state, local election, throwing his support behind a moderate Republican clinging to her state Senate seat in Northern Virginia. At a joint appearance near the Dulles International Airport, Mr. Bloomberg will endorse Jeannemarie Devolites Davis, a state senator from Fairfax whose husband is Rep. Tom Davis, the former chairman of the House Government Reform Committee.

The Devolites Davis campaign announced the endorsement yesterday afternoon.

Ms. Devolites Davis is locked in a tight re-election battle and an endorsement from the well-known Mr. Bloomberg could give her a boost days before the vote next Tuesday.

The key issue is guns, where Ms. Devolites Davis has broken with her party to support increased background checks for gun owners, and the National Rifle Association has even endorsed her Democratic opponent, J. Chapman Petersen.

The mayor’s endorsement “is based on her independence and courage, and putting people over party, especially when it comes to the guns issues,” a source with knowledge of the move said.

Mr. Bloomberg has waged a national campaign over the last year against illegal guns, angering the NRA and many gun owners, including some in Virginia.

Ms. Devolites Davis’s campaign said she has supported efforts to keep criminals from buying weapons at gun shows and has backed allowing local governments to ban firearms on school property. Most importantly for Mr. Bloomberg, however, would be her vote against a bill in the Virginia legislature outlawing the sting operations that New York City conducted against gun shops that engaged in illegal straw purchases.

The district has become more Democratic in recent years, and Ms. Devolites Davis, who has been in the state Senate for one term after serving six years in the Virginia House of Delegates, is said to have tacked toward the center as a result.

Mr. Davis and the mayor have known each other for a long time and said to hold similar views on many issues. The Republican congressman just announced last week that he would not seek the U.S. Senate seat of John Warner, who is retiring. As a moderate, he would have had to go up against a more conservative former governor, James Gilmore, in a primary before likely facing an uphill battle to defeat another former Virginia governor, Mark Warner.

Mr. Bloomberg has made several out-of-state endorsements in recent years, including Governor Schwarzenegger of California, Senator Lieberman of Connecticut, and a moderate Republican congressman in Connecticut, Rep. Christopher Shays. But it is unusual for him to get involved in a state legislature race. “The mayor helps officials who help New York while helping their own constituents,” Mr. Bloomberg’s chief spokesman, Stuart Loeser, said.

While in the Washington, D.C., area, Mr. Bloomberg also will meet with the federal transportation secretary, Mary Peters, to discuss her plan aimed at alleviating delays at John F. Kennedy International Airport. And he will host a reception to unveil a renovated Washington office for the city, which will feature the mayor’s signature “bullpen” layout.

The city’s lobbying arm in Washington has a new director, Bill Daly, as well as a deputy director, Lindsay Ellenbogen. The recent hires and the renovation reflect Mr. Bloomberg’s beefed-up national agenda during his second term, which has fueled rumors — consistently denied — of a presidential bid next year. In addition to gun laws, he has pushed an anti-poverty proposal to expand the earned income tax credit and spoken out on immigration policy.