ALBANY – Senate Republicans are trying to sway the outcome of a Democratic primary in the Bronx by running a “smear campaign” against one of the candidates, Democrats charged yesterday.

Their complaints focus on New York Forward, a fund-raising committee that has spent more than $150,000 on mailings and TV advertisements attacking Assemblyman Jeffrey Klein, the candidate backed by Democratic leaders for the 34th Senate District.

Democrats argue that New York Forward is a front group for the Senate Republicans and their preferred candidate, Assemblyman Stephen Kaufman. They note that the committee raised most of its money from 29 of the 37 Republican senators and appears to share office space with the Senate Republican Campaign Committee.

Messrs. Kaufman and Klein are the leading candidates to replace Senator Guy Velella, who is serving a jail term after pleading guilty to bribery-related charges earlier this year. Although both are Democrats, Mr. Kaufman is expected to join the Republican majority if elected, helping the Senate majority leader, Joseph Bruno, retain control of the upper house of the Legislature.

Democrats said the attacks in the New York Forward advertisements – that Mr. Klein’s law firm defended nursing homes accused of abusing patients – are misleading. The Democrats also accuse Republicans of using the group to circumvent campaign-finance laws, which prohibit party committees, such as the Senate Republican Campaign Committee, from spending money on primary elections.

“We can’t call this action illegal, but certainly the spirit of the law is that state parties should not get involved in party primaries,” Senate Minority Leader David Paterson said at a City Hall news conference yesterday.

Joining him to decry the tactic were other prominent Democrats, including Attorney General Eliot Spitzer, state Comptroller Alan Hevesi, New York City Comptroller William Thompson, and the former president of the Bronx, Fernando Ferrer.

A spokeswoman for New York Forward, Susan Del Percio, said the group operates independently of the Senate Republicans and the candidates it supports, including Mr. Kaufman and Senator Caesar Trunzo of Long Island.

She said the group’s criticisms of Mr. Klein are based on public information from the Web site of his law firm, Klein, Calderoni & Santucci, and records of the state Health Department.

“The voters have a right to know who he’s working for,” Ms. Del Percio said.

Mr. Klein’s campaign manager, Deirdre Schifeling, confirmed that the firm had represented the nursing homes on various legal matters, but denied that it had defended them against charges of abusing patients.

“One of the things that we are deeply concerned about is the distribution of scurrilous charges by anonymous groups – charges that are false, charges that take away from the debate of serious, important substantive issues,” Mr. Spitzer said.

A spokesman for Mr. Kaufman, John Gallagher, said his campaign had nothing to do with the advertisements but does not object to what New York Forward is doing. “There would seem to be some truth to the allegations,” Mr. Gallagher said. “If that’s the case, I think voters have a right to know about it.”

The Kaufman campaign had charges of its own to level, accusing Mr. Klein of improperly campaigning for the Senate using money he originally raised to run for attorney general in 2006. At least some of the contributions he collected for the statewide race exceeded the limit for a Senate race, the Kaufman campaign said.

Ms. Schifeling denied any wrongdoing on Mr. Klein’s part, saying the money he transferred into his Senate campaign account did not include any contribution that exceeded the Senate limit.

A spokesman for Mr. Bruno, John McArdle, confirmed that the majority leader and other senators were among the donors to New York Forward, but said the group was not affiliated with the Senate Republicans.

A photograph provided by Democrats indicates the Albany office suite listed as the mailing address for New York Forward is also the office of the New York State Senate Republican Campaign Committee.