The latest weapon in the U.S. government's sophisticated arsenal aimed at preventing terrorist attacks consists of two police cars, each manned by a single cop, stationed on the U.S. Capitol grounds.

Their mission: to block dangerous-looking vehicles that run security checkpoints and speed toward the Capitol.

The cars are the newest addition to a security system at the once easily accessible Capitol that began last year after a bomb exploded in a corridor 30 feet from the Senate chamber.

"It's just one more step in enhancing security," said U.S. Capitol Police Deputy Chief Harry Grevey.

This step was taken after two cars ran the security checkpoints, for what turned out to be "nonterrorist" purposes, and their drivers were cited for traffic violations, said Grevey, who refused to elaborate on the incidents.

"After that we felt we had to take some other measures," said Senate Sergeant-at-Arms Larry E. Smith, a member of the Capitol Police Board. He said the board is looking into alternate methods, which must be approved by congressional leaders. "None of us are saying that the current plan is ideal."

But for now, the blue-and-white cars, topped by flashers and emblazoned with a Capitol Police seal, sit waiting 24 hours a day at the two driveway entrances to the Capitol grounds, several yards behind a security checkpoint where all drivers must stop on their way in.

Though no one was injured by the Capitol blast in November 1983, it spawned a security system that includes color-coded identification passes, metal detectors, concrete barriers and security checkpoints.

No one wants a repeat of the terrorist attacks in Beirut, in which bomb-laden trucks drove into a Marine compound and the U.S. Embassy grounds, killing nearly 200 people.

But if the potential enemy of the police who man the Capitol Hill squad cars is a bomb-laden terrorist vehicle, their present enemy is boredom.

"This is not exactly my favorite duty," said one officer, who sat in his car for four hours recently with nothing to do "but watch the tourists, watch the reporters, watch the congressmen and watch the squirrels." Officers rotate on what are usually four-hour shifts.

When the cars were set in place two months ago, they kept their motors running constantly, ready to do battle. But officials discovered that this could be "counterproductive," Grevey said.

"When you idle a vehicle constantly and throw it into drive," he added, "it may well stall out on you. It's also terribly hard on the cars."

Now the vehicles sit quietly, their motors running occasionally to warm up their drivers, who plead "no comment" or ask for anonymity when someone inquires about their work.

"They say we should take action to block a vehicle, but not endanger ourselves," one officer explained. "But if a truck comes through with explosives . . . , " he added, shrugging his shoulders and letting his words trail off.

An officer assigned to duties on the Capitol grounds said, "Unfortunately, I don't think the cars could do much good. Realistically, if a terrorist is determined. . . there's nothing you can do to stop him."