Jonathan Weisman and Ashley Parker / New York Times News Service

WASHINGTON — Sen. Harry Reid, D-Nev., took a defiant and uncompromising stand on Monday before a showdown on the future of the filibuster, saying that Republicans must stop blocking executive branch nominations or he will try to change rules to “save the Senate from becoming obsolete.”

But senators emerged from a 31⁄2-hour meeting in the Old Senate Chamber saying they were confident that an agreement could be reached today to defuse the tense partisan standoff, though no deal had been struck in the closed session that went well into the night.

Democratic and Republican leaders promised to continue negotiating, but Reid, the majority leader, said the first test vote was still scheduled for this morning.

“There’s no deal, but there’s a much better understanding,” said Sen. John D. Rockefeller IV, D-W.Va.

All but two of the 100 senators cloistered themselves in the Old Senate Chamber, where some of the great compromises of the early days of the nation were struck and where modern Senates have met at difficult moments, including the impeachment of President Bill Clinton.

Senators said nearly all of them spoke, some passionately, for or against a change to the Senate rules, ostensibly only to end filibusters of presidential nominees for executive branch positions but possibly to pave the way to further limit the filibuster in the future.

Advocates of the change said Democrats would stay the course for a showdown today. “It was a very good discussion, but at this point, we’re headed to votes” that will probably trigger the change, said Sen. Tom Udall, D-N.M. and a leading advocate of weakening the filibuster.