House and Senate leaders voiced strong support yesterday for a set of controversial security proposals that include a plan to build a fence around the U.S. Capitol grounds, as one congressman called the Capitol "an accident waiting to hap- pen."

The congressional leaders and security officials asserted that the Capitol is vulnerable despite guard posts, metal detectors and a complex system of identification badges.

"There are probably 100 different ways to get into the building undetected," said Rep. Trent Lott (R-Miss.), the House minority whip. And Senate Sergeant-at-Arms Ernest E. Garcia said that the Capitol may be "the last public building in town that is not secure."

Still, several congressional leaders predicted an uphill struggle for the $13 million proposal to place 6,800 feet of wrought iron fencing around the grounds and to ban parking in hundreds of coveted spaces on the Capitol's East Plaza. The plan also calls for increased electronic surveillance at the Capitol and the use of sophisticated electronic equipment like that at the White House and other highly secure federal buildings, according to a Senate source.

As to the proposals' chances, Lott, who proclaimed himself a "passionate" supporter of the plans, said, "Members are hesitant to move where they are involved, whether the question is honoraria, pay raises or security. They're not inclined to stand up and take the tough vote."

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said he would support "whatever the experts on the subject of terrorism tell us we should do . . . . What do we hire experts for?"

In the Senate, a spokesman for Majority Leader Robert J. Dole (R-Kan.) said that Dole, who appointed the task force that made the proposals last week, "supports anything that helps security and keeps access what it is now. I think we've accomplished that."

Meanwhile, Murray S. Flander, press secretary to Senate Minority Whip Alan Cranston (D-Calif.), was gearing up the campaign to sell the proposal with a news conference scheduled today and statistics to buttress support for protecting the "7 million visitors who come each year" to the Capitol.

Flander also got the first taste of what he called "the initial negative reaction" to the fence proposal. "One reporter from California kept referring to it as the wall," said Flander. "In his mind it was a wall. It's not."

Indeed, Architect of the Capitol George M. White told a subcommittee of the House Public Works and Transportation Committee yesterday to visualize "the White House fence . . . a wrought iron high fence that would . . . act as a barrier." Garcia later said that the fence would include eight gates, with three for the public and the rest for Hill staff, the press and lobbyists, as well as special provisions for members of Congress.

Garcia said visitors would be screened by guards and metal detectors in enclosures at the gates. Answering criticism that the fence would further restrict access to a building where visitors once roamed freely, Garcia said, "We already have a perimeter, the doors. We're saying extend it to the street. Once you're through it, you'll have much freer access."

But members of Congress already are sensitive to the fine line they must walk between open access to what one called "a worldwide symbol of democracy" and protection against terrorist attacks.

"Politically, it's going to be tough," said Rep. E. Clay Shaw Jr. (R-Fla.), ranking minority member on the House subcommittee that must act on the proposal. "But the most dangerous and craziest among us want to invade our halls. What we are guaranteeing is openness and access for our citizens in a safe and orderly manner."

Subcommittee member Rep. Sherwood L. Boehlert (R-N.Y.) said, "We live in troubled times. The Capitol is an accident waiting to happen."

There was also concern about the price of the proposals in these days of the Gramm-Rudman-Hollings deficit reduction law. Said O'Neill, "If [the House] does it in a hurry, it will be done. If it doesn't, it won't be done. I don't see the funding for it."