Attempts to organize Capitol Hill blue-collar workers, who have fewer job protections than almost any group in America, gained an important ally yesterday as House Speaker Thomas P. (Tip) O'Neill Jr. expressed support for their right to unionize.

But a woman trying to organize the workers was arrested by the U.S. Capitol Police when she refused to leave a room in the subbasement of the Rayburn House Office Building where she was talking to employes on their lunch break.

Congressmen who have supported attempts to unionize some 4,500 blue-collar workers on the Hill were highly critical of the arrest of the organizer, Victoria Lessin, who works for the nonprofit Capitol Employees Organizing Group.

"It's like in the old days when General Motors hired Pinkerton men to keep union organizers out of the GM plant," said Rep. Don Edwards (D-Calif.).

"The Speaker supports the right of employes to bargain collectively, and that includes all federal employes," said Christopher J. Matthews, administrative assistant to O'Neill, when asked the speaker's position on efforts to unionize congressional workers.

The organizing group hopes to bargain for a contract that protects against discrimination and unwarranted layoffs and provides worker health and safety protections. Congress has exempted itself from many labor laws that apply to most American workers, including those guaranteeing the right to unionize.

Eugene A. Servary, superintendent of the House office buildings under the Architect of the Capitol's office, called in Capitol police after Lessin refused to leave the workers' area, a police spokesman said.

Lessin was charged with unlawful entry and remaining on property unlawfully and was later released, said Capitol police spokesmen. She faces a maximum $100 fine and up to 6 months in jail.

Elliott Carroll, executive assistant to the Architect of the Capitol, said Lessin's arrest "had nothing to do with" her attempts to organize workers. She is not a congressional employe and was in a work area not open to the public, Carroll said.

Stuart S. Smith, executive director of the Capitol Employees Organizing Group, said the area is an employes' rest area and they were on break, so Lessin was not interrupting work. "We have a right to talk about their unionization rights," he said.

The Architect of the Capitol has administrative responsibility over congressional employes, but the House Administration Committee makes policy decisions, a committee staff member said.

The organizing group, formed in 1979, failed to unionize Senate restaurant workers when employes voted against it 146 to 35 in a poll by the Senate Rules Committee. Smith charged the poll reflected 3 1/2 years of unfair labor practices on the part of the Senate.

Rep. William Clay (D-Mo.), chairman of the House labor subcommittee, said through a spokesman yesterday that he "deplored" the arrest of Lessin and that he intends to pursue the issue as a right of employes to discuss their work conditions.

Smith said that wages are not as significant an issue as protections for the workers, and that so far the group has signed up for the union a total of 352 workers.

One House restaurant employe, who asked not to be named, said he believes most employes would support unionization but many do not speak up for fear of retribution. "It's a buddy-buddy thing for the top people. They don't promote people who have been in the system for years."