Virginia’s Republican-led Senate on Thursday derailed an effort to eliminate tenure-style job protections for public school teachers, a key part of Gov. Robert F. McDonnell’s education agenda.

The move came after state senators delayed a decision on the contentious measure for eight days. Three Republicans joined all 20 Democrats in voting 23 to 17 to send the bill back to committee, almost certainly killing it for the year.

Had the bill passed, it would have given Virginia school administrators some of the broadest powers in the nation to remove teachers from the classroom.

In a statement, McDonnell called the vote a “disappointing” rejection of a “bipartisan, national movement underway to bring more accountability to our schools.”

The governor promised to continue advocating for his legislation. “Today’s vote is a delay,” he said. “It is not a defeat.”

The Virginia Education Association has opposed the measure, saying it would leave teachers vulnerable to arbitrary dismissal based on personality conflicts or other petty reasons.

“We’re very, very relieved,” said the association’s lobbyist, Robley Jones, after the Senate vote. “What was proposed was just overkill.”

The commonwealth’s teachers spend three years on probation and then receive “continuing contracts,” which are almost always renewed barring unusual circumstances.

McDonnell’s bill would have extended probation to five years and replaced continuing contracts with three-year contracts. At the end of every three years, an administrator could let a teacher go without having to give a reason.

Labor leaders’ opposition to the bill was predictable. But even some avowed tenure-reform advocates have been critical, saying that the measure would allow principals to get rid of a teacher for reasons that have little to do with classroom effectiveness.

“It absolutely plays into what teachers are so afraid of,” said Sandi Jacobs of the National Council on Teacher Quality, a Washington-based group that has become one of the most prominent tenure-reform advocates in the country.

“If the point is,” Jacobs said, “we want to make sure that consistently ineffective teachers don’t get renewed, then why not just say that?”

During floor debate Thursday, Sen. Mark D. Obenshain (R-Harrisonburg) said that under the current system, firing an ineffective teacher is possible, but the process is so time-consuming and difficult that many principals don’t want to go through it.

Bad teachers, Obenshain said — or “lemons” — end up getting passed from one school to another in an unending “lemon dance.”

“We have great public schools across Virginia,” he said. But “if you believe that every teacher in Virginia is a great teacher, you’re wrong.”

Sen. Phillip P. Puckett (D-Russell), a former teacher and principal, said he was offended by Obenshain’s use of the term “lemon.” Teachers need mentorship and support from their supervisors, he said — not the threat of dismissal.

“This bill does nothing but kick teachers in the teeth,” Puckett said.

The motion to recommit the bill, which passed the House last month, was put forth by Sen. Emmett W. Hanger Jr. (R-Augusta). The other two Republicans who voted in favor of sending the bill back to committee were Charles W. Carrico Sr. (Grayson) and Jill Holtzman Vogel (Winchester).

This is the second time this year that senators have debated McDonnell’s bill. The Senate previously rejected its own version of the measure, 20 to 18, with two Republicans defecting. One of them was Majority Leader Thomas K. Norment Jr., whose daughter is a teacher.

Norment did not appear to have changed his mind since that first vote, saying he had no regrets about departing from party line to follow his conscience.

A moderate who has served for two decades, Norment has voted this session with Democrats to help kill several bills championed by conservatives — including the so-called personhood bill, which would have given rights to fertilized eggs.

“You know what?” Norment said. “I sleep very comfortable with those votes at night.”