WASHINGTON — After several years of trying, supporters are on the verge of winning federal approval for a study that would examine whether half a million acres should be added to the Santa Monica Mountains National Recreation Area.

The proposal cleared its last congressional hurdle Tuesday when the House voted 291-117 to give final approval to a comprehensive public lands bill that includes the study.

The legislation now goes to President Bush for his signature.

A White House spokesman said Wednesday the president is expected to sign the bill into law.

The study, which would be conducted by the U.S. Department of the Interior, will look at the suitability of more than doubling the size of the federal recreation area, which straddles the border of Ventura and Los Angeles counties.

The area that could possibly be added to the recreation area is known as the Rim of the Valley Corridor and includes the mountains and adjacent land surrounding the Conejo, Simi, San Fernando, Santa Clarita and Crescenta valleys.

The corridor takes in parts of the Santa Monica, Santa Susana, San Gabriel and Verdugo mountains, as well as the San Rafael Hills and adjacent connector areas to the Los Padres and San Bernardino national forests.

Conservation groups and others say the expansion would create a contiguous trail in the recreation area, which is overseen by the National Park Service.

"This would allow hiking enthusiasts to enjoy greater access to existing trails in the recreation area," said Sen. Dianne Feinstein, D-Calif. "It would preserve the region's fragile chaparral, woodlands and shrubs. And it would protect local wildlife, including mountain lions, bobcats and golden eagles."

Feinstein and Sen. Barbara Boxer, D-Calif., sponsored the legislation in the Senate. Rep. Adam Schiff, D-Pasadena, sponsored the bill in the House.

Land-rights groups and other opponents have expressed concerns that the study could eventually lead to private property owners losing their land.

There are 158,000 landowners in the affected area in Los Angeles County and 11,000 in Ventura County, "and virtually none of them know the train wreck coming at them," said Chuck Cushman, executive director of the American Land Rights Association.

If the area becomes a national park, "their freedoms are going to be lost," Cushman said.

"You're going to have the Park Service closing roads, telling them what color to paint their house. There's just a whole variety of ways their lives are going to be changed. If they have any undeveloped land, they are going to be in jeopardy of ever being able to use that land."

Supporters, however, point out the legislation authorizing the study includes a provision directing the Interior Department to analyze any impact on private land within the affected area or bordering it.

Rep. Lois Capps, D-Santa Barbara, supported the study bill.

"It's important to find ways to balance the preservation and recreational needs of this important ecological area so it can be protected for future generations to enjoy," she said. "This legislation will help us do that."

Rep. Elton Gallegly, R-Simi Valley, voted against the legislation. Gallegly said last year that, while he has supported parks in the past, he is concerned the study could lead to the purchase of additional property.