Thousands of Yale undergraduates are to arrive on campus tomorrow, only to be greeted by an unwelcome but all-too-familiar sight: the ninth labor walkout at Yale in 35 years.

The open-ended strike by janitorial, dining hall and clerical workers is timed to maximize pressure on Yale officials, who insist that a walkout is misguided because, they say, the university has made an unusually generous contract offer.

Yale says it has offered the university's largest union, representing 2,900 clerical workers, raises of 44 percent over the life of a six-year agreement, including immediate raises of 14 percent.

But the clerical union and the union representing 1,100 janitorial, maintenance and dining-hall workers say that the university's offer would leave wages well below those of Harvard's workers and would leave pensions so paltry that many retirees would feel pressured to return to work.

John W. Wilhelm, president of the Hotel Employees and Restaurant Employees International Union, the parent of Yale's two main unions, said he would call off the strike if the university agreed to binding arbitration to settle the dispute.

Mr. Wilhelm angrily criticized the university for rejecting arbitration and for hardly budging in negotiations this week.

"Of all the strikes we ever had at Yale, this is the most unnecessary," said Mr. Wilhelm, a 1967 Yale graduate. "That they won't negotiate and won't accept binding arbitration is the height of irresponsibility."

The unions have been without a contract since January 2002. They held a one-week strike last March.

Yale's unions received some support yesterday from Senator Joseph I. Lieberman, the Connecticut Democrat and presidential candidate and a graduate of Yale College and Law School.

"You're not asking for a lot," he said, appearing alongside union leaders in New Haven. "You're not asking for a fortune, Lord knows."

Standing behind a sign that said, "Break the Cycle of Strikes," Mr. Lieberman added: "I appeal to the leadership of this great university to do what is right, do what is consistent with what you've taught me and generations of others about service and fairness and justice. Don't let there be a strike."

Tom Conroy, a Yale spokesman, said the union was calling a strike at an unfortunate time.

"We regret that the unions have called for a strike," he said. "We don't believe that any disinterested party would look at Yale's offer and conclude that a strike made sense."

He reiterated the university's long-held position against binding arbitration. "The university and the unions should be the ones to agree to what the contract will contain rather than have a third party impose it on us," he said.

Responding to the senator's comments, Mr. Conroy said, "I didn't hear any endorsement from Senator Lieberman of the union's contract proposal as opposed to the university's."

Last Thursday, Yale's president, Richard C. Levin, sent a long letter to the students, faculty, alumni and unionized employees saying that he had tried to improve Yale's rancorous labor relations, widely considered the worst of any university's in the nation. He also insisted that Yale had made a generous proposal, offering raises well above inflation as well as unusual job security provisions.

Dr. Levin's letter discussed pensions at length, largely because that is the most hotly contested issue in the dispute. He said that Yale's proposal would increase pensions 23 to 31 percent and that for employees retiring with 30 years of service at age 65 or older, their after-tax retirement income from their pension and Social Security would be more than 83 percent of their final after-tax salary.

"At a time when employers throughout the nation are seeking givebacks, the university has offered to continue its entire package of benefits," Dr. Levin wrote.

Mr. Wilhelm insisted that Yale's assertions about pensions were distorted because they focused on employees who spent their whole work lives at Yale and who turned down survivor benefits for spouses.

"What they say demonstrates the saying that figures lie and liars figure," Mr. Wilhelm said. "The actual average monthly pension of members of our union who retired from Yale in 2002 with 20 years' service or more is just $621 a month."

The two sides also clashed in comparing workers' pay at Yale and Harvard.

The unions put together a study showing that Yale workers often received 20 percent less, while a consultant hired by Yale, Hewitt Associates, did a study showing that Yale's workers generally earned more than Harvard's after accounting for the higher cost of living around Boston.

Yale said that the average annual pay for its janitorial, maintenance and dining-hall workers was $30,000 and the average for its clerical workers, $33,000.