Virginia Senate Democrats brought a potential defector back into the party fold Tuesday to reject a third state budget plan, killing a two-year, $85 billion proposal because it lacked funding for Metrorail to Dulles International Airport.

The spending plan died in a 20 to 19 vote in the evenly divided Senate after easily passing the Republican-dominated House over the objections of many Democrats. Twenty-one senators must vote for a budget to pass. One senator, John S. Edwards (D-Roanoke), was absent.

Lt. Gov. Bill Bolling (R), who broke many partisan ties during the regular General Assembly session, was powerless to weigh in since he lacks authority to vote on the budget.

The failure of the budget — a surprise since Republicans had seemed to peel off one Democrat during budget talks — could result in a partial government shutdown if a new spending plan is not worked out by July 1, the start of the fiscal year.

Already, the Virginia Department of Transportation has notified contractors that they should plan to halt work by May 1 because of the budget impasse, affecting hundreds of construction projects on bridges, roads and highways across the state.

Gov. Robert F. McDonnell ( R) said the vote will have “disastrous effects on Virginia.”

“This is the most fiscally irresponsible act that I have seen during my career,” a visibly frustrated McDonnell said at a news conference. “It is completely irresponsible to hold up the budget of Virginia that serves 8 million people for one earmark, for one 11-mile railroad in one area of the state.”

Democrats said McDonnell and other Republicans were the irresponsible ones, failing to devote the money needed to solve the region’s traffic woes and ensure that the airport, a huge jobs generator, continues to thrive.

“I am voting no today because this budget does not include enough money for the most important construction project in the commonwealth,” said Senate Minority Leader Richard L. Saslaw (D-Fairfax). “Rail to Dulles is a vital project. . . . The governor says he supports the Dulles rail project. It’s time for him to prove that.”

The Senate will take up the budget legislation again Wednesday, when it meets to vote on bills that McDonnell has vetoed or amended. There was no indication that the results will be any different, although Senate Majority Leader Thomas K. Norment Jr. (R-James City) expressed hope that a Democrat will have a change of heart overnight — even as he accused them of trying to “embarrass and bring down the governor.”

The budget came off the rails during a special session called after Senate Democrats killed two spending plans during the regular session in a dispute over committee power and spending priorities.

The reversal provided yet another moment of drama in a year filled with it, from an opening day dominated by a fight for control of the chamber to pitched battles over abortion, religious freedom and voter ID .

At the center of Tuesday’s hubbub was an 85-year-old man who stands not 5 feet tall and sat quietly drinking Pepsi in his leather swivel chair as Senate colleagues spoke passionately for and against the budget.

Sen. Charles J. Colgan (D-Prince William), a former chairman of the Senate Finance Committee, recently signed off on a budget conference report. In doing so, he essentially endorsed a budget plan that added millions for schools, health care and other Democratic priorities but did not include the $300 million Democrats had sought for Dulles rail.

At the time, Colgan stopped short of saying whether he would vote for the budget, but he seemed to be leaning that way.

“I put a lot of work into the budget — hours and hours, late nights,” he said then. “We can’t just be concerned about Dulles. You can’t worry about this project and not the rest of the budget.”

But Colgan, who said he was torn between the need for a state budget and the desire to support the rail project, said he was undecided even hours before the vote.

“Even walking here this morning, I hadn’t made up my mind,” he said after the vote.

Earlier in the day, Norment said he expected Colgan and another Democrat — he did not say who — to vote for the budget.

Colgan would have made for a curious Democratic turncoat. He is a loyal Democrat who put off retirement in November to run for a record-setting 10th term last year to try to preserve the party’s slim hold on the Senate.

Yet he’s also a social conservative who often votes with Republicans on abortion issues — one who had chewed out both parties on the Senate floor as the regular session wound down for their bitterly partisan tone.

Ever since he signed the conference report, Colgan’s Democratic colleagues had lobbied him hard, going so far as to circulate his cellphone number among Northern Virginia business leaders earlier this week so they could put additional pressure on him. By Monday, his voice mail was so clogged that it had stopped taking messages.

Colgan did some last-minute lobbying of his own, meeting privately with McDonnell Tuesday before the day’s Senate proceedings began.

“What do you want from me?” Colgan said McDonnell had asked him.

Colgan said he asked the governor for a letter expressing support in principle to Dulles rail and to two tunnel projects in Hampton Roads, which had also been a sticking point during budget talks. McDonnell agreed and wrote the letter. That satisfied Colgan, but the rest of the caucus was put off by the governor’s tone.

“They thought it was a little bit testy,” Colgan said. “I didn’t think it was.”

But Colgan nevertheless decided to side with the rest of the Democrats and oppose the budget. He said he called the governor before he cast his vote, ducking out of the chamber during a recess called for that purpose.

“This was a heart-wrenching thing to do,” Colgan said.