Metro riders at the Smithsonian station in 2015. (Astrid Riecken for The Washington Post)

A disagreement between the District and its neighbors over how to split the cost of fixing Metro is blocking the region’s effort to agree on a plan for the transit system, raising doubts about whether a long-term solution can be found this year, officials said.

The argument roiled a meeting of top local officials Wednesday at which D.C. Council Chairman Phil Mendelson (D) argued forcefully against using an existing Metro funding formula to decide what D.C., Maryland and Virginia each would pay.

The city would pay more than its neighbors under that recipe, which Mendelson said would be unfair to the city and inefficient to administer. He also said it would require the District to raise taxes too high.

D.C. instead favors a uniform, regionwide sales tax to raise the additional $500 million a year that Metro says it needs for new equipment and maintenance.

But representatives from Virginia and Maryland fought strongly against Mendelson’s position, saying Metro should stick to the current formula.

D.C. Council Chairman Phil Mendelson, shown in 2015, says the existing Metro funding formula is unfair to the District. (Amanda Voisard for The Washington Post)

Fairfax Board of Supervisors Chairman Sharon Bulova (D) and Montgomery County Council President Roger Berliner (D-Potomac-Bethesda) were among those saying that the Virginia General Assembly would never support the regionwide tax favored by the District, because such a levy would fall disproportionately on Northern Virginia residents and businesses.

The clash underscores the difficulty of forging a consensus days before Metro’s future is to be a principal topic at a regional summit of Virginia Gov. Terry McAuliffe (D), Maryland Gov. Larry Hogan (R) and D.C. Mayor Muriel E. Bowser (D).

Several officials said the argument is worrisome because it shows D.C. might stand in the way of a regional agreement on Metro, whereas the expectation has been that Virginia would be the biggest obstacle.

“The region needs to realize that it’s got to deal with the District’s problem,” Mendelson said in an interview after the meeting of the Metropolitan Washington Council of Government (COG) Metro Strategy Group.

Officials also warned that this dispute, combined with others, could mean the region will fail to agree on a long-term funding plan for Metro in time to win approval from the state legislatures in Richmond and Annapolis in early 2018.

Metro General Manager Paul J. Wiedefeld has said Metro needs the additional funds starting in July 2018 or risks a further deterioration in infrastructure and service.

The COG Metro group, chaired by Bulova, has been working for months to agree on how much money the region needs to raise for Metro and how the financial burden should be shared.

A plan prepared by top county and municipal administrators and budget officers this spring said a one-cent-per-dollar sales tax would generate enough money — $650 million annually — to keep the transit system safe and reliable. It also would generate enough extra money to pay for some expansion of the system, such as construction of a second tunnel under the Potomac River between Rosslyn and Foggy Bottom, which would eliminate one of Metro’s major bottlenecks.

The report said that a sales tax has several advantages over alternatives such as a property tax — it would be easier to administer, would capture revenue from tourists and other nonresidents and is used by other major transit systems throughout the country.

Wednesday’s meeting was closed to the public, and the group’s discussions are normally confidential. Bulova and other participants agreed to discuss it publicly after The Washington Post obtained an account from an official who spoke on the condition of anonymity.

The group made progress on one front. It agreed to endorse Wiedefeld’s request for $500 million in additional funds per year, instead of scaling it back to $400 million as some officials favored.

Virginia state Sen. George L. Barker (D-Fairfax), who is a member of the group but did not attend Wednesday’s meeting, supported the $400 million figure, partly because it would be easier to sell to his Republican colleagues who control the state legislature.

With $400 million a year, Metro could achieve what it calls a “state of good repair” and deploy eight-car trains throughout the system, rather than using six-car trains for some trips.

The additional $100 million a year would allow the transit agency to pay for some additional projects, such as core station improvements, relining the Red Line tunnel to keep water out and upgrading bus service.

“We’re in general agreement that if we’re going to bite the bullet and recommend additional funding, which we know is needed for a state of good repair,,” Bulova said, “then we should also provide a way for us to fund some major projects that we know are going to be needed, so we’re not all sitting down in three or four years and starting all over again.”

The endorsement of the $500 million target will be passed in writing to McAuliffe, Hogan and Bowser, who will gather Monday at Mount Vernon in their first meeting about Metro in 10 months. They also will get an update on the work of former U.S. transportation secretary Ray LaHood, who is trying to develop a plan for the transit agency that can win support from Richmond to Annapolis.

But that agreement was overshadowed by the discord over how much each jurisdiction should contribute.

Under Metro’s funding formula, the District pays 35.7 percent of capital costs, Maryland pays 33.4 percent and Virginia pays 30.9 percent.

The formula, which dates to Metro’s founding 40 years ago, is recalculated regularly to take into account changes in population, ridership and the addition of stations.

Bulova and others proposed using the formula for the new money to be raised. The strategy is that each jurisdiction would then decide on its own whether to raise taxes or find other revenue to pay its share.

But Mendelson, reaffirming an argument the District has made in the past, said the formula is outdated and unfair. He said Virginia, in particular, should pay a bigger share now that the Silver Line has opened.

The first segment of the new line opened three years ago and included five stations — one in Reston and four in Tysons Corner. The second phase, now under construction, consists of six stations — including one at Dulles International Airport and two in Loudoun County — and is expected to open for passenger service in 2020.

“It was designed based on an understanding 40 or 50 years ago of [the original] 100-mile system and a population density that’s very different today,” Mendelson said. “Track miles have grown. The Silver Line was never envisioned.”

He also said using the existing formula would require unacceptable increases in the District’s sales tax or property tax.

For instance, he said, using the existing formula would mean raising the city’s sales tax to 7 percent from 5.75 percent. If a uniform, regionwide sales tax were adopted, it would mean raising the District’s sales tax to only 6.5 percent.

A regional sales tax also would be easier to administer, he said.

“The main argument I stressed is: This is a regional system, and it ought to be paid for on a regional basis,” Mendelson said.

Bulova said a uniform, regional sales tax would unfairly penalize Northern Virginia. A COG technical panel reported previously that Northern Virginia — because of its large population and high volume of sales to be taxed — would pay more than the District and the Maryland suburbs combined.

“That’s not fair to Virginia, and it would never fly in the General Assembly,” Bulova said.

She said the current Metro formula “takes into account stations, ridership, population, and it’s what we’ve been using previously, so why shouldn’t we use it for additional projects?”

Berliner echoed that point.

“The argument for a regional sales tax is its simplicity,” he said. “However, the threshold question is whether it’s fair. We have an existing formula. . . . It is presumably a fair allocation.”