Virginia Senate Democrats, powerless for most of this General Assembly session, summoned the will and strength this week to kill the proposed $85 billion budget.

They voted down their chamber’s spending plan and vowed to do the same to the version heading over from the House. This in a week when two GOP-backed anti­abortion bills unexpectedly imploded.

Democrats feel they’re suddenly on roll. But they shouldn’t count on a comeback just yet, political observers warn.

The budget impasse threatens to consume the second half of the session, just as GOP-backed social legislation did the first.

If Senate Democrats continue not to budge and vote down the House budget, “there’s no vehicle to deal with the budget left,” said Sen. Ryan T. McDougle (R-Hanover). “There would have to be unanimous consent to introduce a new bill or the governor would have to send down a new budget. But there would be no budget bill left in the legislature.”

The General Assembly could adjourn March 10 without a budget — that has happened before — but legislators would have to return for a special session to adopt one before the new fiscal year begins July 1, said Virginia Finance Secretary Richard D. Brown.

And if they didn’t pass a budget by then? Brown said that would be a first.

At least a partial government shutdown would be likely, according to McDougle, who said Republicans were trying to find out whether Gov. Robert F. McDonnell (R) could keep core functions of government — like public safety and prisons — running.

“Unfortunately, we’re looking into that right now,” he said.

Many lawmakers said they were confident it wouldn’t come to a shutdown.

“They’re going to get to a budget,” said House Minority Leader David J. Toscano (D-Charlottesville). “It’s just a question of when.”

If the stalemate does drag on, and seems to do so more because of politics than policy, there could be a backlash, said Bob Holsworth, a retired Virginia Commonwealth University political scientist.

“I think the Democrats have to be careful,” he said. “Since they’ve won the public-relations war this week fairly decisively, I think they have to be careful not to squander it by being perceived as overreaching.”

Until the halfway point of the session, Democrats were unable to stop a host of conservative Republican bills expanding gun rights and restricting voting rights, among other things. They are vastly outnumbered in the House. The Senate was split evenly in November’s elections, but the GOP claimed control because Lt. Gov. Bill Bolling (R) has the power to break tie votes.

But Bolling’s voting power does not extend to the budget. So when the Senate Finance Committee’s budget plan came up for a vote Thursday in the Senate, Democrats found themselves with some muscle.

The Senate voted 20 to 17 for the spending plan, but 21 votes are required for passage. The House passed its budget 79 to 21, with 11 Democrats voting for it. The House budget will make its way to the Senate next week, but Democrats there say they will not vote for that plan, either.

Democrats said they object to the budget because it does not include enough money for such things as schools, social services and mass transit.

“They completely ignored Dulles rail, and the project is vitally important to our long-term continued prosperity in Northern Virginia,” said Sen. Mark R. Herring (D-Loudoun), who also noted that per-pupil spending in kindergarten through 12th grade remains below 2007 levels. “The differences are substantive.”

But with the prospect of a deadlocked Senate looming, Democratic leaders there also have been pushing behind the scenes to reorganize some committees to give their party more power.

Senate Majority Leader Thomas K. Norment (R-James City) went public last week with a letter from Sens. Richard L. Saslaw (D-Fairfax) and A. Donald McEachin (D-Henrico) that asked for more seats on crucial committees. They also requested that the powerful Senate Finance Committee be co-chaired by a Republican and Democrat.

Norment and other Republicans accused them of playing politics with the budget.

“This is not about the policy of the budget,” McDougle said. “This is about individual positions on committees.”

McEachin said Democrats are only trying to get the committee representation they are due. If committee assignments had been proportional to the Senate’s makeup (as was the rule before Republicans scrapped it this year), McEachin said, Democrats could have stopped some legislation, such as a bill to overturn the state’s one-per-month limit on handgun purchases and a measure that, before a last-minute amendment this week, would have required most women to get an invasive type of ultrasound before an abortion.

“If they had organized the Senate along the lines of how people had voted, we never, never would have seen repeal of one-gun-a-month, we never would have seen transvaginal ultrasound,” he said. “Look at the mess they’ve made with all these bills, assuming power they don’t have.”