THE CHARGES against former congressman Henry Helstoski are among the most damaging that can be leveled against a legislator. He is said to have solicited and accepted money from resident aliens in return for introducing bills to improve their immigration status. But it is possible that Mr. Helstoski will never be tried, let alone convicted, for this abuse of office. A provision of the Constitution stands in the way.

That provision says members of Congress "shall not be questioned in any other place" for any speech or debate in either house. The Supreme Court has interpreted those words to protect all legislative actions, including the introduction of legislation. Thus, it said on Monday, the government would be questioning those actions in some other place - a court - if it told a jury Mr. Helstoski actually introduced legislation to help those aliens or if it presented evidence he had told them of his previous successes with private bills for other aliens.

The court did leave an opening. It said the government can try to convince a jury that Mr. Helstoski should be convicted of bribery for promising to introduce those bills in exchange for cash payments. But the problem of sorting out promises from legislative acts turns what is a simple bribery case into a difficult proseution. It guts much of the evidence on which the government has already sent to jail several people, including Mr. Helstoski's brother and one of is former legislative aides.

The court is splitting hairs in its distinctions. The justices are straining logic in an effort to keep on the books some criminal laws that can be used against corrupt congressman. While the Speech and Debate Clause was intended to protect the legislature against vindictive prosecutors and judges, its language and history protect them against most straightforward charges as well.

It would have been cleaner, and better in the long run, if the court had simply ruled that congressmen cannot be prosecuted under ordinary bribery laws for taking money as payment for legislative actions. If it had done so, Congress would now have to face its internal problem. It could, for example, pass legislation waiving the protection of the Speech and Debate Clause in certain kinds of bribery cases; that might or might not be constitutional since the court has carefully reserved the question for future decision. Or Congress could attempt to add criminal sanctions to a rigid code of ethics. Or it could try to police all the activities of its members more vigorously itself. There may be other ways to deal with this problem of congressional corruption. But the court has made it clear that Congress will have to find them.