WASHINGTON — With so much attention focused on Supreme Court nominee Samuel Alito, President Bush’s lower-court nominees have drawn relatively little notice in recent months. But a quiet maneuver last week by Senate Democrats aimed at blocking one of the president’s closest advisers from the federal bench has set the stage for a potentially ferocious battle early next year.

Mr. Bush’s staff secretary and a former clerk to Justice Kennedy, Brett Kavanaugh, is one of six federal 2005 Circuit Court nominees nominated in who have yet to be voted on by the Senate. Mr. Kavanaugh is the only one whose nomination was bounced back to the White House under an arcane rule on Wednesday — and some Republicans think they know why.

A graduate of Yale Law School who clerked for two Circuit Court judges before clerking for Mr. Kennedy, Mr. Kavanaugh, 40, was also an associate counsel in the Office of the Independent Counsel during the 1998 impeachment hearing of President Clinton. He was a principal author of the Starr Report, a detailed account of Mr. Clinton’s affair with a White House intern, Monica Lewinsky.

Mr. Kavanaugh’s supporters in Washington say Mr. Clinton’s wife, Senator Clinton, is behind an effort to scuttle the former impeachment lawyer’s nomination to the federal Court of Appeals for the Washington, D.C., Circuit — a charge that Mrs. Clinton’s office denies.

“I understand there is a relative of former President Clinton in the Senate who has an interest in keeping people out of promotions who at some point engaged in behavior she found unpleasant,” the chairman of the Center for the Rule of Law and a dean emeritus of the Boston University Law School, Ronald Cass, said. “I think what’s happening here is that Senator Clinton has registered an objection to him that is a personal objection.”

Under Senate rules, nominations that have not been voted on in the Senate by the end of the year automatically return to the White House. The Senate typically waives the rule as a way of preventing unnecessary future delays. In the rare cases that it does not, a lone objection is sufficient for the nomination to be sent back to the White House. It is not clear whether one or several senators objected in the case of Mr. Kavanaugh.

A spokesman for Mrs. Clinton, Philippe Reines, dismissed the suggestion that the New York Democrat objected to Mr. Kavanaugh’s nomination. Asked in an e-mail yesterday whether Mrs. Clinton had anything at all to do with the nomination being sent back to the White House, Mr. Reines replied simply: “No.”

A spokesman for a Republican on the Senate Judiciary Committee, Senator Cornyn of Texas, said he does not know who objected to Mr. Kavanaugh’s nomination. He insisted, however, that someone had, and added that he or she should register the objection publicly.

“If there is a problem with the nominee, we should know that, and he should have an opportunity to clear that up,” the spokesman, Donald Stewart, said. “Nobody is publicly saying they are holding him, but somebody is. Otherwise he’d be going through.”