WASHINGTON — Senator Clinton has pleaded with her supporters to back Senator Obama, but one of her most loyal backers is refusing to follow her lead.

Lady Lynn Forester de Rothschild, a top fund-raiser for Mrs. Clinton’s primary candidacy and a member of the Democratic National Committee, is endorsing Senator McCain, warning that Mr. Obama would move the nation “too far to the left” and saying his Republican rival would bring the “bold, bipartisan, centrist change we need in Washington.”

Ms. Rothschild, an entrepreneur and former telecommunications executive, said in a Capitol Hill news conference yesterday afternoon that Mr. McCain had a much longer record of reaching across party lines than Mr. Obama.

Linking the Democratic nominee with “MoveOn.org, Nancy Pelosi, and Howard Dean,” Ms. Rothschild said Mr. Obama was tied too closely to the left-wing base of the Democratic Party. “I am not comfortable there,” she said.

The political center, she said, would be ignored with Mr. Obama in the White House and Democrats like Mrs. Pelosi, the Speaker of the House, running Congress. “The centrist view is not going to have a hearing. I find that very terrifying,” she said.

Ms. Rothschild praised Mr. McCain’s stance on climate change, campaign finance, and stem-cell research, and she criticized as “complete nonsense” the Democratic efforts to paint him as an extension of President Bush. “It is the classic cheap shot. Just not true,” she said.

Ms. Rothschild also praised Mr. McCain’s running mate, Governor Palin of Alaska, although she acknowledged they disagree on some issues, most notably abortion rights. “I think she’s pretty cool,” she said of Mrs. Palin.

Ms. Rothschild said she had not spoken to Mrs. Clinton about her decision to back Mr. McCain. When asked to reconcile her choice with the former first lady’s full-throated endorsement of Mr. Obama, she shrugged. “I don’t reconcile it. We disagree,” she said. She added later: “I’m sure she is not pleased with what I am doing today. But you know what? I have to do what I believe in.”

A spokesman for Mrs. Clinton did not return a request for comment. The McCain campaign said it plans to keep Ms. Rothschild “busy” as a surrogate for the rest of the campaign.

The New Jersey-born Ms. Rothschild is married to the British banking magnate Lord Evelyn de Rothschild and splits her time between London and New York. In backing Mrs. Clinton through the end of the protracted Democratic primary, she made little secret of her personal distaste for Mr. Obama, at one point calling him “an elitist.”

During the news conference, she appeared acutely aware that her wealth and status as a British baroness makes her an imperfect surrogate in a campaign increasingly focused on economic populism. While noting that she had married into “a very grand family,” she took pains to stress her middle-class roots, describing her childhood in “a typical American town” in New Jersey and a father who worked two jobs.