SOMEHOW, when the Senate is forced to deal with the issue of its own pay, everything seems to get terribly complicated. The concerned citizen may perhaps be forgiven for suspecting that behind some of these complications lies a desire on the part of at least a few senators to keep their options open so that, the next time no one seems to be looking, they can restore the kind of backdoor pay arrangement that they currently enjoy.

On June 9 the Senate surprised almost everyone when it voted a 30 percent limit on outside income and honoraria, on which there is now no limit. The same 30 percent cap now applies in the House. But at the same time the Senate voted against a pay raise to the House level of $69,800. Reconsidering that vote a week later, the Senate finally voted itself a pay raise, but in so doing it postponed the effective date of the 30 percent cap on honoraria until Jan. 1, 1984.

Does that sound like a Solomonic compromise? Or does it sound like an attempt to allow senators to earn extra thousands this year and then, at some later moment when the spotlight is off, to repeal that 30 percent cap before it goes into effect?

We think the Senate did the right thing when it followed the House and voted to raise its own salary. Only by voting regularly for pay raises, notwithstanding the political fallout, will members of Congress be able to keep their pay from falling way behind inflation. But there should be a cap on outside honoraria. These payments come, after all, not from taxpayers but from sources with a keen interest in special legislation. The basic $69,800 salary, with the extra 30 percent in outside earnings allowed by the House rule, adds up to $90,740--surely, adequate compensation.

The measure will now go to conference. The House, with all due deference to senatorial prerogatives, should insist that the senators apply the same effective date to the cap on outside earnings that applies to the pay raise. The backdoor should not be left ajar.