WE HAD THOUGHT that the expansionist impulses on Capitol Hill had been quelled for this year after Congress decided not to rebuild the West Front and the House dropped its plan for yet another parking lot. But we were wrong. The Senate has just voted to extend the Capitol - not the building, this time, but the grounds.

Specifically, S. 1859, which was whisked through the Senate Wednesday without debate or dissent, would annex several streets around the edges of Congress's 181-acre domain. What's the point? According to the Senate Rules Committee's report, "Capitol Police jurisdiction over traffic and other matters on the Capitol grounds for security purposes would be strengthened." That's a little vague. As Post staff writer Jack Eisen has reported, though, the annexation would have at least two concrete effects.

First, Congress would take over the areas along Maryland and Pennsylvania Avenues NW where food and souvenir vendors are now allowed to peddle their wares. The vendors got permission to sell at the foot of the Hill, you may recall, last year after the National Park Service kicked them off the Mall. But if S. 1859 goes through, they'll have to move again - because vending is forbidden on the Capitol grounds. We can't imagine a good reason for pushing these enterprising merchants out; surely their sales to visitors don't hurt the business of Congress's own snack bars, which cater mainly to staff anyway. So this aspect of the annexation seems unfair and unwarranted.

Second, by acquiring these streets, Congress would also get more parking places - at least 200 more, by our reporter's count. That may well be what "traffic and other matters" really means. The grab will not make too much difference to visitors because those spaces, while not off limits for the general public, are usually occupied by congressional staff cars.

What does matter is that, once again, Congress would be trying to ease its own parking snarls by preempting more space. Sen. Gary Hart (D-Colo.) had a much better idea the other day when he proposed imposing fees at congressional parking lots. According to Sen. Hart, a survey of Hill employees found that 44 per cent of those polled would take the Metro if parking charges were in force. That would be good for Metro and Capitol congestion and clean air besides. But abolishing free parking is still a heretical notion on the Hill. No one will be surprised to hear that the Senate voted to shelve Mr. Hart's proposal. Perhaps the real surprise is that it managed to get 28 votes.