The nation's most expensive and meanest Senate race entered its final round today with another escalation of mudslinging.

A new poll showed Sen. Jesse Helms (R-N.C.) holding a 3 percent lead over Gov. James B. Hunt Jr.

Amid signs that his election hopes are slipping, Hunt seized the offensive in a series of news conferences around the state. He accused Helms, who is seeking a third term, of "trying to buy reelection with tainted money" by engaging in a "Watergate-like cover-up" of a Federal Election Commission probe of his political machine.

Hunt allies also charged that three Republican U.S. district attorneys had "politicized" the Justice Department and the FBI by launching a crackdown of voter fraud on the eve of the election.

Hunt forces said the crackdown, announced at a news conference today, was designed to "intimidate" Democratic voters.

"Gov. Hunt is behind in the polls and he's grasping at straws," said Helms' Press Secretary Claude Allen. "He is desperate."

A George Gallup poll, commissioned by several North Carolina newspapers and television stations, showed Helms leading Hunt 49 to 46 percent.

It indicated that Hunt has been unable to cut into Helms' lead since mid-September when another Gallup poll showed Helms leading 49 to 44 percent. Hunt and Helms have been trying to play down the importance of polls in the closing days of their campaigns.

"Our polls show this race within 2 or 3 points," Hunt said today. "I think the election is in a dead heat."

Hunt's charges today centered around a two-year-old complaint to the FEC by Rep. Charles G. Rose (D-N.C.) about the operations of the National Congressional Club, a political action committee set up by Helms' allies and Jefferson Marketing, an advertising firm also run by Helms supporters.

Several newspapers, quoting sources, reported today that FEC General Counsel Charles M. Steele recommended Oct. 16 that the commission find the two organizations in violation of federal elections law for providing cut-rate services to campaigns, which amounted to illegal corporate contributions.

The FEC considers such recommendations to be private internal communications, and officials refused to confirm news accounts.

But Hunt said the reports substantiate that Jefferson Marketing is a "dummy corporation" set up to make "illegal and excessive contributions" to Helms and other conservatives.

"Now we know why Sen. Helms has been so anxious to cover up the truth about his political machine," said Hunt, 47, a two-term governor who has mounted the most serious challenge ever faced by Helms, 63.

Hunt charged that Helms, who has gone to court to block the FEC probe, has acted like President Richard M. Nixon in the matter.

"Just as Richard Nixon managed to keep the Watergate scandal under wraps until after the 1972 election, Jesse Helms apparently has won his battle to hide the truth from the people . . . until after the 1984 election," Hunt said.

He estimated that Helms, who has raised more than $13 million, has received more than $1 million in cut-rate advertising services from Jefferson Marketing. "Sen. Helms is trying to buy reelection with tainted money, and he's trying to cover up his federal investigation into the wrongdoing of his political machine," the governor said.

The Hunt campaign later called a second news conference to denounce an anti-vote fraud effort by the state's three Republican U.S. district attorneys, Samuel T. Currin, Kenneth W. McAllister and Charles R. Brewer.

In a statement distributed to reporters in Hunt's behalf, former U.S. attorney general Griffin Bell said he was "shocked" that the Justice Department is "apparently engaging in a partisan political effort in North Carolina."

Currin, a former Helms aide, and the other two district attorneys denied political motivation in their efforts.