ST. LOUIS (AP) -- John Danforth has advised the Interior secretary not to count on his family foundation's offer of $50 million -- at least for now -- to fund construction of any cultural attraction that may be built on the Gateway Arch grounds.

Danforth, a former Republican senator and U.S. ambassador to the United Nations, wrote the "heads-up" letter to Interior Secretary Dirk Kempthorne late last week, foundation spokesman Paul Wagman said Tuesday. Danforth advised that the foundation's previous $50 million offer would be a "very significant challenge" to fulfill given the stock market's decline, Wagman said.

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The private foundation, which works to revitalize the St. Louis region, still wants to provide financial support for a cultural attraction that meets certain criteria. But it now says it will have to consider a request for funds in light of its assets at the time such a proposal is made.

The foundation's most recent public filing in May 2007 listed $223 million in assets. But that figure does not reflect subsequent grants and the stock market's recent decline, said Wagman, who would not provide a current total.

The foundation's financial support has been contingent upon the National Park Service building a major, above-ground, architecturally significant cultural attraction that is centrally located on the 91 acres of National Park Service land surrounding the Arch.

But it's not clear that's what the Park Service envisions. It is considering a design competition to revitalize the Arch grounds, but has said any changes would have to conform to the property's status as a historical landmark.

Park officials said last month they favored a plan that would increase educational opportunities and possibly improve or add to visitor amenities, like a restaurant.

The nonprofit National Parks Conservation Association has expressed concerns about potential development, and has urged the Park Service to strike a balance between improving services and preserving the site's National Historic Landmark designation.

Deputy superintendent Frank Mares has said that recent public hearings generated an expressed desire to link the Arch grounds more aesthetically and practically to the city's downtown, perhaps through improved pedestrian walkways and a more welcoming entrance.

The Park Service's Tom Bradley, who oversees the Jefferson National Expansion Memorial in St. Louis that includes the Arch grounds, did not return a phone call seeking comment.

The Danforth Foundation already has spent $2 million on studies exploring ways to change the look of the riverfront and Arch grounds. Danforth himself has been leading the effort to re-energize the area surrounding the monument.

In July, a "Groundswell for Change" friends group, Web site and steering committee were formed to urge the National Park Service to enliven the Arch grounds and riverfront, and connect them to downtown.

Architect Eero Saarinen designed the 630-foot monument for a 1947 competition. It was completed in 1965, and remains the city's icon.

Don Castleberry, a former Midwest regional director of the National Park Service, argued Sunday in a St. Louis Post-Dispatch opinion piece against transferring national park lands to a private entity such as the Danforth Foundation.

He wrote the Arch belongs to all Americans and should be protected against encroachment "in every possible way."

(Copyright 2008 by The Associated Press. All Rights Reserved.)