With Clinton gone, Democrats are without another member at a time when they hold a 51-49 majority in the chamber. Sen. Edward Kennedy (D-Mass.) is away being treated for brain cancer; Sen. Robert Byrd (D-W.Va.), the Appropriations Committee chairman, was hospitalized earlier this month, the third time this year; and Sen. Barack Obama (D-Ill.) is pushing ahead on the presidential campaign trail with little time to spare for Capitol Hill.

That won’t change things too much in the short run, given that the Senate requires 60 votes to move controversial legislation.

Clinton, the first serious female contender for a major party’s presidential nomination, ended her campaign on June 7 at a speech at the National Building Museum in Washington. She then abruptly canceled her public schedule and has gone into a period of seclusion.

She had formally opened her campaign with a website announcement establishing an exploratory committee on Jan. 20, 2007, and led the Democratic field in polls throughout 2007. But she finished only in third place in the Iowa caucus in January and, despite winning the New Hampshire primary soon thereafter, was unable to prevent a steady shift of votes and delegates to Obama’s column.

Few people saw Clinton as a long-term senator after her initial election in 2000. It was widely assumed that she would make a run for the presidency. Now that that goal appears to have eluded her, at least temporarily, there is equally widespread speculation about what her next goal will be. A run for the governorship of New York state has often been mentioned, and another presidential bid in 2012 has also been discussed, with Obama’s defeat this year as a prerequisite.

At least one Clinton supporter suggested Clinton’s absence may be prolonged. Sen. Barbara Mikulski (D-Md.) said Clinton would be “joining us later this summer,” when asked about her return. Reines said this referred to a public event Mikulski and Clinton will both be attending later in the summer.

Manu Raju contributed to this article.