How many times will the Washing-ton area have to summon U.S. Transpor­tation Secretary Ray LaHood to referee our squabbles before we finally agree to extend the Silver Line to Dulles International Airport?

The region’s quarreling children called in Daddy LaHood for a second time last week. It helped, for sure. He seems to have broken the impasse over a labor issue that threatens to scuttle the project’s Phase 2, which would build Metrorail from Reston to the airport and farther into Loudoun County.

But despite all the happy talk about progress after top-level meetings hosted by LaHood on Wednesday, two additional formidable obstacles remain to actually getting this thing built.

One, which hasn’t attracted much attention, is a federal audit underway into the Metropolitan Washington Airports Authority. The multi-state agency, known as MWAA, is overseeing construction of the Silver Line.

The audit won’t be completed for months, but an initial report will be released May 15. That’s just in time to influence a key vote by the Loudoun County Board of Supervisors on whether to pull out of the project.

It’s plausible that the audit will reveal or at least highlight past problems in MWAA’s complicated, fractious management structure. The investigators, from the inspector general’s office of LaHood’s department, are also looking at the potentially explosive question of whether the airports authority is correctly projecting how high tolls will rise on the Dulles Toll Road to help pay for the line.

Troubles on either front would hand fresh ammunition to enemies of the project, especially some conservative politicians in Richmond and Loudoun.

“The Republicans are licking their chops, expecting something to come out . . . to discredit MWAA,” said a Northern Virginia Democratic official, who spoke on the condition of anonymity because the issue is politically sensitive.

For the sake of the project, I’m hoping that its defenders will succeed in arguing that the audit deals mostly with problems from years ago that don’t involve the Silver Line.

Supporters, including some Republicans, emphasize that management of the airports authority has dramatically improved with the arrival in July of a new chief executive, John Potter, a former U.S. postmaster general; and in January of a new board chairman, Michael Curto, a Washington lawyer.

“I think that Curto and Potter are really turning MWAA around,” said Tom Davis, a former Republican congressman from Fairfax County who is now vice chairman of the airports authority board. “We still have some difficult challenges on Phase 2, even if everything goes right. But I think they are getting the board focused. Prior to this, we had regional wars. They’re trying to bring this thing back on track.”

The other big obstacle is cost. The undecided votes on the closely divided Loudoun Board are worried both about tolls on the Dulles Toll Road and extra costs to Loudoun taxpayers.

I agree with supporters, such as the Loudoun business establishment, who say that benefits from growth around new Metrorail stations will easily cover the cost in the long term.

But Board Vice Chairman Janet S. Clarke (R-Blue Ridge), who hasn’t made up her mind about the Silver Line, worries that the rewards will come too slowly.

“My point is, can we afford to front the money, and for how long will we have to do so before we start to receive the income from the economic development,” Clarke said.

The good news is that LaHood’s latest intervention nudged the airports authority, the state and Loudoun toward resolving the dispute over a pro-union labor provision in the Phase 2 construction agreement.

The parties apparently are headed to a compromise in which the airports authority will drop or significantly soften the labor provision. In return, the state won’t demand additional concessions before releasing $150 million that it’s pledged to the project.

The issue is largely symbolic but politically important to pro-union activists in the Democratic Party and anti-union ones in the GOP.

(If the pro-union provision stays, Loudoun’s all-Republican board will pull out of the Silver Line for sure. For more on this issue, see my colleague Steve Pearlstein’s column in the Sunday Business section.)

LaHood had to get involved last year, as well, when internal disputes over costs threatened to kill the project. He brokered a deal that trimmed the price tag by about $1 billion.

The secretary plans to retire at the end of this year, so we probably can’t depend on him to bail us out a third time. If our region is finally going to achieve the eminently desirable goal of linking our biggest airport to our principal mass-transit system, then we will have to mature sufficiently to solve these problems for ourselves.

To read previous columns by Robert McCartney, go to /mccartney.