Virginia Gov. Terry McAuliffe said Thursday that Maryland Gov. Larry Hogan's proposal for funding Metro falls short of a long-term fix for the transit agency's financial difficulties, but he left open the possibility that the plan could serve as the basis for an interim solution.

In a letter to Hogan (R), McAuliffe (D) joined D.C. Mayor Muriel E. Bowser (D) in faulting the plan because it does not provide for a new tax or other permanent, dedicated revenue source for Metro.

On Monday, Hogan proposed to raise $2 billion over four years in additional funding for Metro, with equal contributions from Maryland, Virginia, the District and the federal government. He ruled out raising taxes in Maryland, which means there would be no quick adoption of a mechanism for dedicated funding that Metro long has sought.

McAuliffe's letter singled out that missing element, saying to Hogan, "We must both ensure that this [financial] commitment is long-term and the funds are dedicated, so that [Metro] is able to finance its current and future capital needs."

The letter, a copy of which was obtained by The Washington Post, also expressed concern Metro funding would be delayed by Hogan's insistence that the federal government increase its contribution to the transit agency. It urged progress toward changes in Metro's governance, including creation of a smaller "reform board" to replace the current 16-member panel.

But McAuliffe welcomed Hogan's new willingness to increase Annapolis's contribution to Metro. A spokesman for McAuliffe said that while the governor saw dedicated funding as a vital goal for the future, it was not a necessary condition for a short-term plan to raise additional funds for the agency.

"He views this [Hogan plan] as a positive first step, but he wants to keep working to meet the needs of this system," McAuliffe spokesman Brian Coy said.

The criticisms from McAuliffe and Bowser of Hogan's proposal show the region's top officials are still at odds over Metro funding, two and a half weeks after they clashed over the issue in a regional summit .

In response to McAuliffe's letter, Hogan's office said the Virginian should focus on building a regional consensus.

"Instead of endless delaying and searching for reasons to say 'no,' we strongly encourage Governor McAuliffe to join with Maryland and officially commit to this additional funding for Metro," Hogan spokesman Doug Mayer said. "Now is the time for all three jurisdictions to join together and bring a strong proposal to the federal government."

Mayer also endorsed McAuliffe's call to reform the Metro board, "starting with the current chairman [Jack Evans] on down."

Bowser did not respond to requests for comment on the McAuliffe letter. Her office has not provided a detailed response to the Hogan proposal, apart from a two-sentence statement Monday that without dedicated funding, Metro cannot meet the goals of improving safety, reliability and capacity.

But McAuliffe's apparent openness to a compromise adds to momentum in the region in favor of a short-term solution, in recognition that anything more ambitious is politically impossible.

Several top county officials from the Virginia and Maryland suburbs said this week that they too are open to a version of the Hogan plan, providing that the four years were used to develop a permanent solution.

Speaking Wednesday at a regularly scheduled board meeting of the Metropolitan Washington Council of Governments, Fairfax County Board of Supervisors Chair Sharon Bulova (D) welcomed Hogan's proposal as "significant" progress.

"It really helps to move the needle and provides incentive to the District and Virginia to continue to try to find a way to provide the funding that is needed," said Bulova, who chairs COG's Metro Strategy Group.

Metro General Manager Paul J. Wiedefeld also said this week that Hogan's plan may be the best available alternative for the agency until after gubernatorial elections in Virginia in November and Maryland next year.

Underlining the urgency of the debate, Wiedefeld briefed his board Thursday about the need to cover a projected $7.5 billion shortfall in capital funds over the next 10 years, or risk deterioration in safety and reliability.

Asked how Metro would fare if it doesn't get the additional money for new rail cars, other equipment and maintenance, Wiedefeld said: "Think of the last five-plus years. And think of that getting worse and worse."

The McAuliffe missive responded to the surprise move by Hogan on Monday, where he sent McAuliffe and Bowser a letter in which he dropped what had been staunch opposition to giving more money to Metro.

McAuliffe welcomed Hogan's change of heart, saying, "I am glad you now also support new funding for [Metro]."

But McAuliffe expressed concern about Hogan's insistence any package include an increased contribution from the federal government. Metro board members and the region's congressional delegation have said that could be impossible, given the desire in both the White House and much of Congress to reduce federal transit spending.

"We recognize that in the current political environment, [increased federal funding] will take time, but are hopeful if we present a united regional position it will help in our efforts with Congress," McAuliffe wrote.

At the COG meeting, several officials suggested Virginia, Maryland and the District may have to contribute money, as provided in the Hogan plan, without requiring the federal government match it.

"Whether the federal government is going to pony up another $125 million [a year], we can all make a personal guess," Montgomery County Council President Roger Berliner (D-Potomac-Bethesda) said.

But Berliner suggested the three jurisdictions could each put in an extra $125 million a year, for a total of $375 million, so "let's lock that piece in."

However, it's unclear where the three would get the prescribed contribution, let alone additional money to cover the federal government's share.

Politically speaking, McAuliffe is in a weak position to advocate for more funding for Metro. He leaves office in January after one term, as dictated by Virginia law. He has said before that he supported dedicated funding of some sort, but only after Metro showed improvements in safety, reliability and efficiency.

The Republican-controlled legislature in Richmond, whose leaders come from outside Northern Virginia, has resisted raising taxes or increasing state spending on Metro.

Metro board member Michael Goldman, who represents Maryland, said Hogan doesn't think dedicated funding could pass in the legislative session that begins in January, as the region hopes.

"I think it's fairly clear to the governor that because of the elections in Virginia, and essentially Maryland is ramping up to 2018 being an election year, that nothing is going to come out of the Maryland legislature, in the three months that it'll meet at the beginning of 2018."

Goldman said the "sweet spot" for progress on Metro would be in 2019, after the elections are completed.

Faiz Siddiqui and Martine Powers contributed to this report. This story has been updated.