WASHINGTON - The Senate headed for a bitter conclusion to an often-acrimonious year, with Democrats vowing to remain in session until they confirmed Janet Yellen as chairwoman of the Federal Reserve Bank along with other pending nominees, despite delaying tactics by Republicans who are upset over new filibuster restrictions.

Rather than adjourn Thursday, as many had hoped, the Senate was due to remain in session overnight and probably hold a rare weekend vote after Republicans spurned a request from Majority Leader Harry Reid (D., Nev.) to defer pending nominations into the new year without requiring President Obama to resubmit them, as would normally be required.

Because Democrats last month changed rules to prevent filibusters for most judicial nominees, Republicans lack the ability to block confirmations. But they are using other Senate rules to delay the process long enough to cut into Democrats' Christmas recess.

As a result, Democrats will be forced to hold late-night, early morning and weekend votes in the coming days, or face the prospect of starting the process from scratch for nominees who remain unconfirmed by the end of the session.

Most Republicans are already departing Washington to start their vacations, with several staying behind so at least one member can be present in the chamber to object if Democrats try to expedite the process by waiving rules that require up to 30 hours of debate before a final confirmation vote can occur.

Democrats will be left largely alone in the chamber in the coming days to vote on Yellen, a new IRS commissioner and various key deputies for departments, including Alejandro Mayorkas, the former federal prosecutor from Los Angeles nominated to be the No. 2 official at the Department of Homeland Security.

"I'm leaving tomorrow night," said Sen. Lisa Murkowski (R., Alaska). "I don't care what the schedule is. I'm going to be with my family."

Republicans said their actions are justified because Democrats changed the Senate rule on filibusters with a simple majority vote. "What they did was reprehensible, so this is a necessary approach," said Sen. Orrin G. Hatch (R., Utah.).