HELENA, Mont. - Montana's congressional delegation has yet to endorse Gov. Brian Schweitzer's plan to create a brucellosis "buffer zone" around Yellowstone National Park, even as livestock officials struggle to push a renegade group of bison back into the park.

Schweitzer proposes that any outbreaks of the disease on ranches in an area that extends about 50 miles from the park would not count against the state's brucellosis-free status.

The issue has taken higher profile since the state's first outbreak of the disease, which causes cattle to abort. That problem has been compounded by roughly 300 bison that are refusing to leave summertime cattle range and return to the park.

The renegade bison, including nearly 100 calves born this spring, were being hazed back into the park Friday with uncertain results.

Bison carry the disease, as do some other wild animals, and ranchers want to make sure they don't transfer a second case of brucellosis to a domestic cattle herd. The state loses its coveted disease-free status if that happens.

The latest round of brucellosis test results, on blood samples drawn from cattle near the original infected herd in Bridger, came back negative Thursday. State livestock officials have said testing could continue at least through the next month.

Schweitzer has said his proposed buffer zone would protect the Montana cattle industry from the National Park Service's mismanagement of the bison herd in Yellowstone. He said it doesn't make sense to force ranchers far from Yellowstone to worry about a disease their cattle have little risk of contracting.

But Rep. Denny Rehberg, R-Mont., said the disease should be stopped at the Yellowstone border. Extending an artificial zone might just lead to decisions down the road to extend the zone even farther.

"Where does that end? You end up being 100 miles, 300 miles then 500 miles out," said Rehberg, a rancher himself. "If we are interested in a buffer zone … it should exist inside of Yellowstone Park."

Rehberg said he is seeking more money to find a vaccine to eradicate the disease. In the meantime, state and federal officials need to strictly enforce the mutual agreement that calls for the slaughter of bison that don't stay inside Yellowstone.

Some ranchers in the zone proposed by Schweitzer, a Democrat, say they would be unfairly subjected to mandatory testing of their animals and would be forever tainted by the disease.

Hal Harper, Schweitzer's chief policy adviser, said the management zone is just one idea until a method is found to eradicate the disease from the park. He said it is a "fundamental misunderstanding" to say the buffer zone would be extended outward.

Harper noted that elk, which travel much farther than bison and aren't controlled like bison, also carry the disease. The proposed management zone takes that into consideration.

U.S. Sen. Jon Tester's office issued a statement saying he supports Schweitzer, but stopped short of supporting the idea for a "buffer zone."

"The governor has demonstrated a positive approach to solving several difficult issues here," Tester spokesman Matt McKenna said. "We will be working with his office and the relevant federal agencies to find a long-term solution to this problem."

U.S. Sen. Max Baucus' office said the Democrat is "intrigued" by Schweitzer's idea and is looking into it. The senator also supports more money to develop a vaccine for the disease.

"In the short term, (Baucus) would like to see the Interior Department take a more aggressive approach to managing bison near the park," spokesman Barrett Kaiser said.

Many ranchers at a Tuesday meeting of the Board of Livestock came prepared to talk about the proposed management zone.

Paradise Valley rancher Justin O'Hair said the zone would divide ranchers against each other, and in some cases even split apart family ranches.

"We already have a boundary around Yellowstone National Park, and that is where we ought to draw the line," he told livestock officials.

The meeting, though, turned its focus to the renegade herd of bison that are refusing to return to the park despite numerous hazing attempts.

That issue has become more difficult because of the calves, some just a week old, and is drawing intense interest from opponents to bison slaughters.

Harper said the state is investigating ways to avoid killing the calves, a potential public relations nightmare, should the bison herd start wandering back out of the park again over the weekend or next week.

"It would be great if the bison cooperate this time," Harper said. "The protection of this state's cattle industry is a top priority."