The Senate Rules Committee tentatively voted yesterday to poll Senate restaurant and cafeteria workers to see if they want to be represented by a union.

If the Senate eventually recognizes and bargains with an employes' union, it will be the first such action by either house of Congress.

Yesterday's action, which will not be final until absent committee members vote, authorizes only a preference poll of the workers that will not be binding on the committee or on the Senate, said committee staff director John B. Childers.

But the vote does represent a hairline crack in a wall Congress has built around itself to keep its employment practices from being reviewed or questioned.

Congress over the years has approved strong laws ensuring the rights of workers to form unions, outlawing various kinds of unfair labor practices and prohibiting discrimination based on race and sex--but has exempted itself from all of them.

If the poll is finally approved by the committee, the more than 200 Senate employes who work in the cafeterias and restaurants in the Capitol Building and Senate office buildings also will be asked if they want to remain as Senate employes or if they would rather work for a private contractor.

Cafeteria employes formed the Capitol Employees Organizing Group in 1979, and the next year 130 workers signed up for union representation. Along with wanting to bargain for better pay, workers have contended that they are subject to arbitrary firings and that promotions are based on race.

When Congress refused to deal with them collectively, the group last year took its grievances to the International Labor Organization, an arm of the United Nations.

"I welcome the fact that the Senate is finally doing something about the situation . . . but the results of an election at this point would be meaningless because the employes have been intimidated," said Stu Smith, executive director of the employes' group. "We think the appropriate action is for the Senate to start contract negotiations" with the CEOG.

Smith contended that the employes have been led to believe that if they vote for a union the Senate will simply contract out the work to a private company and they will lose their jobs.

Senate Rules Committee Chairman Charles McC. Mathias Jr. (R-Md.) made it clear at yesterday's meeting that the poll would be done by secret ballot to prevent fear of reprisals, a spokeswoman for Mathias said.

Voting to take the poll were Mathias and Sens. Wendell H. Ford (D-Ky.), Daniel K. Inouye (D-Hawaii) and Dennis DeConcini (D-Ariz.), Childers said. Eight other members will be polled in the next few days.