Democrats persuaded 85-year-old Charles J. Colgan to run for a record-setting 10th term last year — just one more time, they said — to try to preserve the party’s slim hold on the Virginia Senate.

Now, Colgan is the reason Democrats could end up losing their biggest fight in Richmond this year: a budget deal that will cost the extension of Metrorail to Dulles International Airport $300 million.

Colgan (D-Prince William) signed off on the compromise Friday even though it meant bucking his own party and providing hundreds of millions of dollars less for one of the nation’s largest public infrastructure projects.

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

Most House and Senate budget negotiators agreed to the two-year, $85 billion spending plan, paving the way for an April 17 vote by all 140 members of the General Assembly.

Colgan stopped short of saying whether he will vote for the budget, which would almost certainly ensure its passage, but said he was pleased with the compromise. “It’s not a guarantee,” he said. “I’ll make up my mind in another two weeks.”

The proposal pumps tens of millions more into local governments, public schools, hospitals and nursing homes than what Gov. Robert F. McDonnell (R) recommended in December.

State employees, college faculty and some local workers would get a one-time 3 percent bonus in December and a 2 percent raise — their first since 2007 — in 2013 if revenues remain at projected levels.

The plan also provides funding for a half-dozen Northern Virginia projects that Colgan wanted, including $2 million for a performing arts center at George Mason University, $800,000 for a community college building in Loudoun County and $400,000 for a program that helps people with disabilities.

November’s election left the Senate evenly divided, but Republicans took control because Lt. Gov. Bill Bolling (R) breaks tie votes. Bolling, however, can’t vote on the budget.

Senate Democrats had voted down two budgets and threatened to kill a third after Republicans rebuffed their request for increased spending in certain areas and more power on committees.

But in recent weeks, as the General Assembly kicked off a special session on the budget, Republicans agreed to fund more Democratic priorities and Democrats agreed to divorce the budget process from their demand for more power in the chamber.

“It’s a pretty tough budget to vote against,” said House Majority Leader M. Kirkland Cox (R-Colonial Heights). “I would be surprised if it didn’t pass.”

Still, Democrats held out hope that Colgan will reject the plan. “There ain’t a deal,” said Senate Minority Leader Richard L. Saslaw (D-Fairfax). “I don’t see Chuck breaking with our caucus.”

Colgan, a former World War II pilot and the longest-serving state senator in Virginia history, had decided before each of his past three elections that he was going to retire, but he was persuaded to run again by Democratic leaders, including former governor Timothy M. Kaine and Sen. Mark R. Warner.

A moderate Democrat and committed Catholic, he sometimes crosses party lines to vote with Republicans, particularly on abortion issues, but he has been a stalwart Democratic vote on fiscal issues.

Bob Holsworth, a former professor at Virginia Commonwealth University who closely follows state politics, said that Colgan is the least likely to hold the budget hostage to political considerations.

“Colgan is typically not a political game player that would be engaged in some effort to manipulate the process or to intentionally misdirect people about this intentions,” he said. “He’s a pretty straight shooter.”

Colgan said he was not worried about fellow Democrats’ reactions to his vote. “They’ve been mad at me before,” he said.

And, he said, he was not promised anything for his vote, although Republicans previously offered to let him co-chair the powerful Finance Committee along with Sen. Walter Stosch (R-Henrico) or simply be known as chairman emeritus. “I’m pretty happy where I am,” he said.

Failure to pass a budget by July 1, the start of the fiscal year, could result in a partial government shutdown for the first time in Virginia history.

Democrats secured several of their priorities but failed to get House negotiators to provide an additional $300 million for Dulles rail after McDonnell’s administration said it would not spend more than $150 million. Transportation Secretary Sean T. Connaughton did not return a call seeking comment Friday.

“There’s a lot of good things in the budget that we have negotiated, and it’s too bad that the administration’s failure to address the needs of Northern Virginia is creating this havoc,” Sen. Janet D. Howell (D-Fairfax) said.

The Senate budget had included bonds to help hold down tolls in Northern Virginia, which are due to be increased to pay for the second phase of Metro’s new Silver Line. That decision could jeopardize the project, according to some, though a more broadly held view is that it will still move forward but result in heavy toll increases.

“I think we’ve got the best deal that we can get right now,” Sen. John Watkins (R-Powhatan) said. “It’s irresponsible to put a single issue like that as the stumbling block for an entire budget.”

The first phase of the $6 billion Silver Line is under construction from Falls Church to Reston and is expected to open in late 2013 or early 2014. Construction on the second part of the project, which will run to Dulles and into Loudoun, is expected to start in January and be completed in five years.

To help pay for the project, a one-way trip that now costs $2.25 could increase to $4.50 as early as next year. By 2018, tolls for that one-way trip could rise to $6.75.

But even some of the Senate Republicans, who had signed off on a budget plan that included the $300 million, said they were never comfortable with the idea of issuing bonds for toll abatement.

“It is bad policy to be going [around] borrowing money which the state must pay back with interest to do toll abatements,” said Senate Majority Leader Thomas K. Norment (R-James City).

Legislators will return to Richmond on April 17 for a vote on the budget, a day before they act on McDonnell’s amendments to other bills passed during the regular 60-day session.

If the budget passes, McDonnell will have seven days to amend, sign or veto it. If it fails, legislators will start over.