A new campaign is pinning the city's mounting subway problems on construction unions.

"They are primarily focused with scoring the most expensive contracts they can for themselves," Luka Ladan, a spokesman for the Center for Union Facts, said in a phone interview. "But in the process, they're leaving subway riders behind with a dirty, delayed, and dangerous subway system."

The Center for Union Facts is a shadowy Washington-based group that has long campaigned against organized labor. This month, it began targeting the unions that staff the MTA's major capital projects, such as the construction of the Second Avenue Subway.

Their new "Subway Scam" campaign charges that "expensive work rules and cost overruns" drive up the MTA's expenses at the expense of maintenance.

"We've seen subway transit construction costs in New York run seven times higher than the global average," Ladan said.

The center won't say who is paying for the campaign, which features a website, billboards, and full-page newspaper ads. The campaign singles out Gary LaBarbera, the head of the Building and Construction Trades Council.

The council declined to comment on allegations of overstaffing and other waste, but it blasted the Center for Union Facts as "a front group for anti-union forces."

"This pseudo campaign is replete with lies, half-truths, and a repulsive anti-union, anti-worker agenda. The cowards funding this effort should stop hiding," the council said in a statement.

A recent New York Times series, as well as a study by the Regional Plan Association, highlighted the soaring construction costs associated with transit projects in the city.

"In New York, you've got 25 people on a tunnel boring machine," said Nicole Gelinas, a senior fellow at the Manhattan Institute. "In Western European countries, you may have seven people on the same machine, you may have as few as five."

MTA officials said they are looking at how to save money in its construction program.

"We can't ask the public to provide more money for our transit system, even though it's desperately needed, unless we can prove that we can invest it efficiently and appropriately," MTA Board Member Scott Rechler said.

So far, the MTA has not come up with any specific proposals to rein in union costs.