Plans to build a $300 million annex for the National Air and Space Museum in Northern Virginia are being reconsidered by the Smithsonian Institution, which is under pressure from a senior House member and a campaign to move the facility to Denver.

In what Virginia officials say is a "serious challenge" to the annex's proposed site at Dulles International Airport, Rep. Sidney R. Yates (D-Ill.) has begun a congressional inquiry into the museum proposal. At Yates's urging, Smithsonian officials recently traveled to Denver and conferred with leaders of a local group working to win the facility.

Yates's move also may renew an intense battle between Virginia and Maryland over the museum. Maryland officials tried unsuccessfully to locate the facility at Baltimore Washington International Airport. But Yates plans to conduct hearings in February that he predicts will be "a major showdown" among the jurisdictions that want the museum.

Virginia officials say Yates's initiative effectively reopens a three-way bidding war they thought they won almost a year ago. In January, the Smithsonian's governing body, the Board of Regents, decided to locate the museum on 180 acres in Loudoun County adjacent to Dulles.

But Congress, through its control of the Smithsonian's budget, has the power to overrule the regents. Yates chairs the House Appropriations subcomittee that oversees the Smithsonian's finances.

"We are dismayed by what has been occurring," said Glenn K. Davidson, Washington liaison for Virginia Gov. L. Douglas Wilder. "At what point does the decision become final?"

Yates said the proposed Denver location might cost the federal government less than the one in Northern Virginia, and questioned whether Smithsonian officials seriously considered locations outside the Washington area.

"I don't think they ever looked at {Denver}, because I don't think they want to go to Colorado," Yates said. "Denver says they can do it for half the cost of Northern Virginia. That immediately perked my interest."

The museum annex has been pursued intensely because leaders in the competing jurisdictions consider it a potential economic and political bonanza. The annex would display a vast collection of items for which the main museum has no room. Among them are the space shuttle Enterprise and the Enola Gay, the World War II aircraft that dropped the first atomic bomb.

Officials predict the annex could draw millions of tourists, promoting growth and enhancing the prestige of the area around it. Citing these advantages, one congressional aide predicted that Virginia, Maryland and Colorado will "fight until the ribbon is cut" on a completed structure.

Martin Harwit, the director of the Air and Space Museum, led the delegation of museum administrators that visited Denver this month. He said the group went because "Mr. Yates suggested the Denver site be given a hearing, and we wanted to inform ourselves about what the people in Denver were offering.

"The regents of the institution have not said anything about this" since they approved the Dulles location, Harwit said. "But certainly, Mr. Yates has to be taken very seriously."

The Denver group wants to locate the facility at Stapleton International Airport, which will be closed when construction of a new airport is complete. Although Denver entered the competition late and was considered the third-place finisher in January, the group has continued its work and plans to present a considerably improved offer to Yates.

"We feel the competition has never been closed," said Rep. David E. Skaggs (D-Colo.), who is leading the Denver efforts. "This is a very serious undertaking to which the best talent in the Denver area has been applied."

Denver has set up a private nonprofit corportion to woo the museum, obtained millions of dollars in corporate pledges and is preparing a local financing package to help pay construction costs. The group also maintains that lower labor costs in the Denver area would allow the project to built for less money there.

Virginia's winning bid in January included sizable financial incentives, including free land for the museum, a $3 million interest-free loan and $100 million in bond financing for the project. Maryland put forward a similar offer.

Virginia officials contend that it would be prohibitively expensive and perhaps impossible to move the Smithsonian's collection of aircraft to Denver. The Board of Regents also emphasized a desire to build the facility in the Washington area, where it would be accessible to the millions of visitors to the main Air and Space Museum.

The Maryland site meets that criterion, but Maryland has not attempted to reopen the competition since January, a decision that Virginia officials called "walking away in a gentlemanly fashion." But that is expected to change. Paul E. Schurick, a spokesman for Gov. William Donald Schaefer, said Maryland would "relish the opportunity" to try again. Yates said Rep. Benjamin L. Cardin (D-Md.) already has inquired about presenting a proposal at the hearing.