THE SENATE surprised almost everyone, including most senators, when it voted Thursday to impose a limit on the amount of outside income in general and honoraria for speeches in particular that its members could earn. Since 1981, senators have been free to pocket all the honoraria they can get, even though predictably many such payments come from groups interested in legislation; some earned more in outside income than in their salaries. Now they are limited to 30 percent of their salaries, the same percentage that applies to members of the House. Congratulations should go to Sen. Henry Jackson (D-Wash.), who sponsored the cap, and to the 50 other senators who voted for it.

But the fight is not over. Before the Senate voted for the 30 percent cap, it voted resoundingly against increasing its salary from $60,662 to $69,800. The House, in contrast, voted itself such an increase last December. So now senators don't make as much money as representatives. That strikes us as anomalous, but we wouldn't advise you to make any little jokes about it to a senator right now: many of them have very little sense of humor on the subject.

Fortunately, tomorrow the Senate will have a chance to vote on an amendment, also sponsored by Sen. Jackson, to raise the Senate salary to the House level. We think they should. Of course, there is some political risk here, but House members have already acted, and they all come up for reelection every two years. Moreover, there is a good case to be made for a salary increase. Congressional salaries did not rise at all for several years, because members fiddled around trying to devise automatic mechanisms for raising them rather than taking the responsibility and voting on the issue. It is plain now that the only way members of Congress will get reasonable pay increases is by taking the political heat and voting for them regularly. Senators might as well get in the habit now.

The Senate would make a mistake, however, if it should raise its salary above the House level. A $69,800 salary plus 30 percent outside income adds up to $90,740. That may not be as lavish an income, given a senator's financial obligations, as many voters assume, but it isn't starvation wages either. Nor do we think the Senate should adopt a proposal to give each senator the option of taking the pay increase with the cap on honoraria or of keeping the lower salary but with no cap on outside earnings. But so long as any senators are earning substantial shares of their incomes from private lobbies with special legislative interests, the reputations of all senators will suffer. The Senate, having had the wisdom to impose a 30 percent limit on its outside earnings, should now have the political courage to raise its salary to the level already voted by the House.