RTD should guarantee that a potential sales tax bailout for FasTracks will be used only for the lines that are threatened with cutbacks, Aurora Mayor Ed Tauer proposed Monday at a meeting of the Metro Mayors' Caucus.

While the mayors have yet to decide whether to seek a public vote on a second FasTracks sales tax hike, Tauer said any newfound money should be dedicated exclusively for the FasTracks projects that the Regional Transportation District says are endangered by the current shortfall.

Tauer and other mayors at Monday's meeting also said RTD should provide a guarantee that this would be the last time it needs more money for FasTracks.

"I can't imagine a scenario where you get the support you need for a tax increase but we don't get a better guarantee than we got the last time," Tauer told RTD staffers at the session.

RTD says the current tax and finance plan can build the lines to Denver International Airport, Arvada-Wheat Ridge, Lakewood-Golden and some other FasTracks elements.

Tauer said if that's the case, the mayors should insist that any new revenue go to the threatened lines. Those would be the commuter-rail lines to Boulder-Longmont and Commerce City-Thornton, plus a light-rail corridor in Aurora and extensions of existing light rail to Lone Tree, Highlands Ranch and Five Points.

But Thornton Mayor Erik Hansen said that if an election were held on a metrowide tax increase dedicated only to projects in the north and east metro areas, it might be a hard sell to many voters.

The entire FasTracks program, originally budgeted at $4.7 billion, is now estimated at $7.9 billion - though RTD expects that number to fall later this month when recent drops in commodities prices are factored in.