A group of private citizens, in fact, is now quietly forming to consider alternatives for the long-range financial security of the facility. No names are yet available.

The trust, created to cover construction debt and build a reserve for operations and maintenance, stood at $109.3 million in December 2005 but fell to $100.1 million as of March 14.

The fund's decline threatens the viability of a complex plan to repay Overture's construction debt. If the trust falls much more, financial "firewalls" must be used to meet debt payments. City taxpayers, however, appear to have no looming liability.

But the fund's decline reduces the money available for upkeep of the high-use, high-maintenance facility. The trust is supposed to deliver $1.4 million annually to the reserve but provided only $900,000 in 2007 and will give an unknown sum this year.

Mayor Dave Cieslewicz, who met with city financial and legal staff to discuss the situation Wednesday afternoon, "is concerned," spokesman George Twigg said. "We are close to the point where the firewalls could come into play."

But the city doesn't manage the trust, which is controlled by an independent trust fund board, Twigg said.

Dana Chabot, treasurer of the Madison Cultural Arts District, the quasi-public entity that runs Overture Center, said the situation is "serious," but that it's best to stay with a current long-term investment strategy.

Overture Center is doing all right in terms of ticket sales and operations and will be able to meet its budget this year, Chabot and Twigg said.

But there is concern for the future if there is no trust-fund-fueled reserve to serve as a backstop, Chabot said.

Overture was built with a $205 million gift from philanthropist W. Jerome Frautschi. Initially, half the gift was put in a trust for operations and maintenance, while the rest was coupled with a $115 million loan to build the structure.

But the initial plan didn't work, and in late 2005, the City Council approved a controversial refinancing deal.

Under the deal, the trust covers about $7.45 million in annual debt payments plus $1.4 million annually for the reserve to the arts district.

But the trust hasn't earned the 8.25 percent needed to meet all commitments.

"This is exactly the fear I had when the council debated the financing plan," said Ald. Zach Brandon, 7th District, an arts district board member. "It's time we stop doing what we've been doing and try to figure out a better solution."

The trust, which must be at least $104 million to fully meet commitments, fell below that sum last summer and has not topped it since.

When the trust falls between $104 million and $100 million — where it is now — it can cover debt payments but only partial payments to the reserve, like the $900,000 in 2007.

The $900,000 given last year, however, is being used for operations and for short-term maintenance this year, Chabot said. The arts district has a $5 million reserve, but it's being held as a firewall for debt payments. In 2013, the arts district will need about $1.5 million annually to do major maintenance on the facility.

If the trust falls between $100 million and $97 million, financial firewalls cover part of debt payments — Frautschi the first $5 million and then the arts district, with the city's initial liability now largely passed. No payment would be made to the reserve.

If the trust falls below $97 million, creditors and the trust fund board would evaluate the need for change.

"While there is no immediate worry Overture will be able to fulfill its mission, these developments reinforce the concerns many of us have had about the long-term financial viability of the center," Chabot said.