Senate Republicans are on the brink of losing control of the state Senate, the last stronghold of the party’s power in the state, after suffering a devastating loss in a contest yesterday for an open seat.

The loss was a momentous victory for Governor Spitzer, whose caustic relationship with Republicans has taken a heavy toll on his agenda, and came as a shock to Majority Leader Joseph Bruno, whose party was defeated in a heavily Republican district that was supposed to be safe territory.

It leaves both the future of Mr. Bruno’s 13-year leadership of the Senate and the more than four-decade-long Republican hold over the Senate in peril.

With 100% of the ballots counted, the Democratic candidate, Assemblyman Darrel Aubertine, defeated his Republican challenger, Assemblyman William Barclay, with 52% of the vote. The vote differential was about 2,500.

The Republican lead in the Senate now stands at one seat, 32-30, approaching a fall election season in which several Republican incumbents are expected to face highly competitive challengers in districts that lean Democratic.

Mr. Bruno, 78, who last year enjoyed a revival by casting himself as a victim of Mr. Spitzer’s aggressive tactics, faces the gravest challenge of a political career marked by overcoming tough odds.

For Mr. Spitzer, the Democratic victory could prove to be a major turning point in his young administration, much of whose agenda has been stalled by clashes with Mr. Bruno that often turned personal.

Since taking office, Mr. Spitzer has sought to dislodge Republicans from power and turn control over to a more liberal Democratic leadership that he expects to unite behind his legislative priorities.

On a range of key issues, Democrats in the Senate, who are led by a representative of Queens, Malcolm Smith, differ with Republicans. Passage of legislation legalizing gay marriage, strengthening rent control regulations in New York City, and scaling back Rockefeller drug laws would all become more likely under Democratic leadership.

Yesterday’s loss is certain to put Mr. Bruno’s standing as majority leader in jeopardy, raising concerns among his colleagues, some of whom have long wanted his job, about his ability to carry the Republicans past the November elections.

A Republican lobbyist interviewed before the race said a loss may persuade Mr. Bruno, who has given no indication of retiring, to decide against seeking re-election.

On Monday, Mr. Bruno, who represents Rensselaer County, warned of the “ramifications” of turning over a seat to a Democrat who will “be part of the downstate coalition” and giving “Eliot Spitzer total control of the government of New York State.”

The majority of Democrats in the Senate are from New York City, the hometown of both the governor, a resident of the Upper East Side, and Speaker Sheldon Silver, a resident of Lower Manhattan.

An emboldened Mr. Spitzer is widely expected to ratchet up his efforts to topple the majority before the November elections by trying to recruit at least one Republican member to switch parties.

“I extend my congratulations to Darrel and commend both candidates on a hard fought race. Darrel will be a powerful advocate for the North Country, and I look forward to working together on the peoples’ business,” Mr. Spitzer said in a statement last night.

Privately, Democrats say they are hoping that the upset will lead to more retirements of Republicans, a quarter of whom are 73 years old or older.

In recent weeks, Republicans have been jolted by the sudden retirement of two long-standing senators and reports of a renewed federal investigation into Mr. Bruno’s business interests, prompting many in Albany to wonder if the conference’s days were numbered.

Republicans had been hoping that yesterday’s election would have given them a chance to reassure anxious donors and interest group allies, particularly the state’s largest health employees union, 1199/SEIU, whose backing the senators have relied on to survive in an increasingly Democratic state.

Mr. Aubertine, a low-key farmer from Jefferson County who has served in the Assembly since 2002, began the race as an underdog against Mr. Barclay, a scion of one of the region’s wealthiest families, in a district where registered Republicans outnumber Democrats 78,000 to 46,000.

While Mr. Spitzer did not campaign in the district, his presence was clearly felt. Through the state Democratic Party, Mr. Spitzer poured hundreds of thousands of dollars into the Aubertine campaign and put his political advisers in charge of polling and advertising.

Raising money primarily from donors outside the 48th district, which encompasses parts of Central New York and the North Country region, the campaigns together spent almost $4 million (or about $77 a vote), an unusually high amount for a Senate race.

The race was heavy with negative advertising, with Mr. Barclay accusing Mr. Aubertine of being a pawn of Mr. Spitzer and Mr. Aubertine portraying his opponent as an aristocrat who is out of touch with the common voter. Mr. Barclay, the son of a former state senator, carried endorsements from many of the state’s largest labor unions.

In one ad, which received wide attention in the area, Mr. Aubertine attacked Mr. Barclay for charging fisherman a fee to fish on a section of the Salmon River that runs across property owned by the Barclay family.