Sen. Phillip P. Puckett (D-Russell) speaks for his "Sunday hunting" bill during the floor session of the Virginia Senate inside the State Capitol in Richmond, Va., Friday, Feb. 7, 2014. (BOB BROWN/AP)

Virginia Republicans snatched control of the state Senate on Monday, immediately ending a budget stalemate by pushing Democrats to agree to pass a spending plan without Medicaid expansion, Gov. Terry McAuliffe’s top priority.

The power shift forced Senate Democrats to yield after a protracted standoff that had threatened to shut down state government in less than a month, according to several lawmakers with direct knowledge of the deal. Democratic negotiators agreed in a closed-door meeting Monday to pass a budget without expanding health coverage to 400,000 low-income Virginians.

The developments saddled McAuliffe with a General Assembly fully in the hands of a party fiercely opposed to his agenda.

The news followed the unexpected resignation of a fellow Democrat, a senator from southwest Virginia, who pulled himself out of the running for a state job Monday amid claims that he had traded his seat for the position. Phillip P. Puckett’s surprise exit from the Senate could doom McAuliffe’s legislative agenda for his remaining 31 / 2 years, political observers said.

With no allies in power in the Capitol, McAuliffe will have to sidestep a recalcitrant legislature, perhaps by turning to executive orders, to achieve his priorities, which, aside from expanding Medicaid, include job creation and expanding abortion rights and gay rights.

McAuliffe has already attempted to single-handedly roll back restrictions on abortion clinics and has explored ways to expand Medicaid without legislative approval. Now, he risks further inflaming partisan tensions in a place that has long prided itself on conducting the people’s business in the more collegial “Virginia way.”

“It really leaves him without an institutional ally,” said former Virginia Commonwealth University professor Bob Holsworth. “He’s going to have difficulty in terms of putting forward a governorship that’s defined by a big legislative agenda. That’s not going to happen.”

Longtime GOP strategist Ray Allen said the governor could figure out how to work with Republicans and seek common ground — as Democrat Mark R. Warner did as governor a decade ago. If not, Allen said, it’s going to be a tough few years.

“The dynamic in this town since January has been 2 to 1, basically the liberal Senate and liberal governor and the conservative House,” Allen said. “Now it’s going to go 2 to 1 the other way.”

McAuliffe spokesman Brian Coy declined to comment “on a deal that has not been fully fleshed out for us.”

“Obviously the governor continues to believe the best thing for the Virginia budget, its citizenry and its economy is using federal money to cover as many as 400,000 Virginians and create as many as 30,000 jobs,” Coy said.

McAuliffe finds himself in this position seven months after his party swept all three statewide offices for the first time in 24 years. Democrats, feeling like they were on a roll in a state growing bluer each year, were stunned by the abrupt change in fortune.

“We lose control of all the [Senate] committees. We lose control of the body. We have worked like hell to get that control,” said Sen. J. Chapman “Chap” Petersen (D-Fairfax), noting victories in two special elections and the election of Democratic Lt. Gov. Ralph S. Northam, who presides over the chamber and has the power to break tie votes. “We jumped through a lot of hoops, spent a lot of money. Now, if you believe the reports, it’s all going out the window.”

Puckett’s resignation turns a Senate that had been evenly split into a chamber where the GOP, at least temporarily, has a 20-to-19 majority. And because Puckett’s rural district leans more Republican than when he first took office in 1998, Democrats will have a hard time hanging onto the seat in a special election.

Republicans say they will pass a budget without Medicaid expansion before Puckett’s seat is filled.

Under a rule that Senate Clerk Susan Clarke Schaar said had not been exercised in at least 40 years, Republicans on Monday used their new sway to call the Senate back into session Thursday. They needed nine senators to call for the session; 21 signed on, including one Democrat, Sen. Charles J. Colgan (D-Prince William), who has split with his party before to end a budget showdown.

Republicans hope to pass the new budget deal out of both chambers that day and immediately send it to McAuliffe’s desk. Democrats still hope for a separate special session to address Medicaid expansion.

News of Puckett’s pending resignation began leaking Sunday, along with word that he was expected to get a job as deputy director of the Virginia Tobacco Indemnification and Community Revitalization Commission — and that his daughter was in line for a judgeship.

Puckett withdrew his name from consideration for the tobacco commission job Monday amid an uproar over what Democrats called an unseemly if not illegal quid pro quo. Critics said Republicans were dangling the jobs to get Puckett out of the Senate and give them a leg up in a months-long budget-and-Medicaid stalemate, which threatens to shut down state government if it is not resolved by July 1.

Puckett issued a statement Monday acknowledging that he was resigning, in part, to allow his daughter, Martha Puckett Ketron, to serve as a juvenile court judge. It was not, he said, because the judgeship was being offered as part of a back-room deal.

Puckett’s service in the Senate was a well-known impediment to his daughter’s judicial appointment because the Senate has a policy against seating the relatives of sitting legislators. The House, which does not share that policy, has twice approved her for the post, which Ketron already holds on a temporary basis.

“My colleagues on both sides of the aisle acknowledge that she is fully qualified for the position,” he wrote. “At this point in my life, I feel that I cannot allow my political career to hamper my daughter’s future and her desire to serve the families and children of our area on the Juvenile and Domestic Relations Court.”

Puckett disputed the idea that he was resigning to accept a job with the commission. He said he was stepping aside in part for unspecified “difficult issues” facing his family. Several people close to Puckett said he had confided that a health issue is involved.

Although many Democrats criticized Puckett for resigning in the middle of the budget and Medicaid battle, Sen. Richard L. Saslaw (D-Fairfax) defended him, saying that a “conspiracy theory” concerning his exit was inaccurate and that media reports missed what was really going on.

“There’s some personal issues involved,” Saslaw said, though he did not want to describe them. “There was never any deal, okay?”

Several other people with direct knowledge of Puckett’s actions said otherwise, however.

Del. Terry G. Kilgore (R-Scott), chairman of the tobacco commission, said Sunday that its executive committee was expected to meet and consider appointing Puckett, perhaps as early as Wednesday.

Early Monday afternoon, the commission canceled a meeting scheduled for Wednesday. The only item that had been on the agenda was “discussion and consideration of a prospective candidate for employment.”

A liberal activist group, ProgressVA, called for a criminal investigation into the alleged resignation-for-jobs deal. But a lawyer familiar with public corruption cases doubted there was anything illegal even if the jobs were explicitly offered in exchange for his resignation. In addition, Virginia Attorney General Mark R. Herring announced Monday that there would be no “investigative role” for his office.

“It may look crass and it may look underhanded and political, but that doesn’t make it a crime,” said Stanley Brand, a former counsel to the U.S. House of Representatives and a criminal defense lawyer.

Puckett and his wife cleaned out his Senate office late Sunday afternoon. He asked Schaar, the Senate clerk, to meet him there, and he presented her with his resignation letter.

He and Schaar also chatted briefly about how after winning a special election to the Senate, Puckett asked Schaar to swear him in at the courthouse in his home district, eight or nine hours by car from Richmond. She flew in the governor’s plane to the event, which 200 to 300 people came to watch.

“They were just delighted that someone from Richmond would come out there,” she said.

Hours before news of Puckett’s departure started to leak, Schaar said the senator and his wife quietly sorted through his office belongings, deciding what to keep and what to toss, and then started out for the long trip home.

“They had just collected trash cans from all over the GAB [General Assembly Building] and piled stuff in,” she said. “I got a cart in the basement for them, and we took it to his car . . . and they were gone.”