It was Super Bowl Sunday. In a Nassau County hamlet, the Imbriale family was watching the game on a big-screen television when the kitchen phone rang.

On the other end of the line was a recording of a young woman’s voice — fast-talking, cheerful, and dull — reminding them to vote for Maureen O’Connell, the Republican state Senate candidate running in the high-profile 7th District special election, two days away.

John Imbriale, a Republican, thought nothing of it and hung up. Fifteen minutes later, the phone rang again. The recording had changed, but the voice was the same, this time telling him that Ms. O’Connell has “worked for years to expand health coverage and believes we should have lower taxes.”

By the time the Colts won, the Imbriales had received four more automatic calls, known as robo-calls. Their Hicksville home was one of perhaps thousands in the Nassau area that fended off an onslaught of robo-calls that night.

The next day, a frantic Ms. O’Connell sent an e-mail to supporters assuring them that her campaign was not responsible for something as unwise as harassing voters during the big game. She blamed the calls on her Democratic opponent, Craig Johnson, an accusation he denied. She also said she was filing a complaint with the U.S. attorney for the Eastern District of New York.

As it turned out, the Super Bowl calls were not the only suspicious episode in the race, according to Senate Republicans. Three weeks after Mr. Johnson, a county legislator, defeated Ms. O’Connell and put the Democrats one seat closer to gaining control of the chamber, Republicans say they are using their legal staff to piece together evidence that links Democrats to at least four dirty tricks that took place during the last week of the pivotal race.

Allegations of dirty tricks are a regular feature of New York politics and fly both ways. Last November, Democrats said Republicans were responsible for the pre-election wave of deceptive robo-calls that went out to districts across the state.

In this new round of accusations, the stakes and tensions are higher. Republicans, who are hanging on to their Senate majority by a two-seat thread, say they seek to pin the alleged tricks on Governor Spitzer, who played a major role in the race, recruiting Mr. Johnson, helping to bankroll his campaign, and directing his own campaign manager, Ryan Toohey, who now works for Global Strategy Group, to provide guidance. (The Working Families Party did much of the fieldwork on behalf of Mr. Johnson.)

“The so-called Spitzer reform team has brought politics to an all-time low,” the deputy Republican majority leader of the Senate, Dean Skelos, told The New York Sun. “Members of our conference have to be ready for this type of low-level campaign. You just have to be prepared it.”

Republicans say GOP residents in the 7th District received another set of robo-calls telling them that Ms. O’Connell, who started career as a registered nurse at North Shore University Hospital, performed abortions. Mr. Skelos said the accusation was untrue.

Nassau households, which are sensitive about the construction upheaval that would be caused by a Long Island Rail Road third track project, were also told by a robo-caller that Ms. O’Connell had accepted a $10,000 donation from the Metropolitan Transportation Authority, Republicans say. Mr. Skelos, who represents a nearby Nassau district, said Ms. O’Connell did not receive any such money from the authority, which is not permitted by law to give political donations.

Republicans in the district said they also received a flier in the mail sent by a group billing itself as “Democrats for Maureen O’Connell.” The flier, which was obtained by the Sun, described how the candidate is “strongly supported by SEIU/1199, the mega union,” has “pledged to fight against cutting the state budget,” and has embraced Roe v. Wade. The flier was signed by Abigail Steen, whose name does not appear in a local phone directory.

“It’s a new low in dirty, misleading campaigning. Quite frankly, the dirty tricks really crossed the line. I’ve never seen such a despicable negative campaign of untruths and lies,” Mr. Skelos said.

Mr. Toohey and Mr. Johnson’s campaign manager, Brian Stedge-Stroud, deny any involvement in tricks. Mr. Stedge-Stroud suggested in an interview that Republicans were suffering from a bad case of sour grapes and were looking for excuses to explain away Ms. O’Connell’s loss. Mr. Johnson won the February 6 race by almost eight percentage points.

“The campaign had nothing to do with it,” Mr. Stedge-Stroud said. “They’re just trying to muddy up the waters because they’re trying to hide the fact they’re losing on issues, such as property taxes and reforming Albany.”

Mr. Skelos said federal investigators had not responded to Ms. O’Connell’s complaint. He said his office would go over finance records from the Johnson campaign submitted in the next mandatory filing period.

“Somebody paid for it,” he said. “It’s not something that just happens.”