ALBANY, N.Y.  —  A former Pentagon official under President Reagan said Monday she will seek the Republican nomination to challenge Democratic Sen. Hillary Rodham Clinton's re-election bid this year.

"I am running to be my party's nominee for the United States Senate from the state of New York," Kathleen Troia "KT" McFarland wrote in a letter to supporters. "Toward that end, I have formed an exploratory committee, which I expect to upgrade to a full Senate campaign committee in the coming days."

The move by the political neophyte, under consideration for several weeks, could put her into a September primary against former Yonkers Mayor John Spencer, already endorsed by top state Conservative Party leaders. No Republican has won statewide office in New York without Conservative Party support since 1974.

Clinton, asked Friday about a possible challenge from McFarland, said: "I am going to remain focused on my job and my race. I will leave it to the other side to sort out their own candidacies."

Polls have shown Clinton far ahead of her potential Senate challengers and leading among possible candidates for the 2008 Democratic presidential nomination.

McFarland, a former National Security Council staffer, has never run for public office and has been out of the public sector since she left her job as a deputy assistant secretary of defense for public affairs in 1985, a post she had held for three years, to raise a family.

Spencer's supporters have in the past dismissed McFarland, a supporter of abortion rights, as an "elitist liberal."

McFarland said she plans to stress her expertise in defense and security issues.

"We are confronted with a dangerous and determined enemy abroad and political divisions at home that are eroding our national spirit," she wrote. "At a time when Americans should be working most closely together, we are being pulled apart by partisan rancor and counterproductive 'gotcha' politics."

State Republican leaders originally settled on Jeanine Pirro as their Clinton challenger, but Pirro, then Westchester County district attorney, pulled out of the race in December, unable to raise much money or interest in her candidacy.