Metro is losing $400,000 a day as the federal government shutdown drags on, cutting into its ridership and parking revenue, according to a letter the agency sent to the region’s U.S. senators.

The transit agency, which estimates federal workers make up 40 percent of its rush-hour ridership, says it is suffering steep losses amid the shutdown — the longest in U.S. government history. In the letter, which was tweeted by Sen. Mark R. Warner (D-Va.), Metro General Manager Paul J. Wiedefeld writes that the agency has suffered daily rail ridership losses averaging 16 percent; average daily ridership for Metrobus is down 8 percent.

The losses, he said, could become more acute as the shutdown stretches on because February SmartBenefits — the commuter benefit offered to federal workers to incentivize transit use — will not be distributed for the month if the shutdown stretches beyond Jan. 21.

“Our preliminary analysis estimates that for an average weekday when the federal government is closed, Metro is losing approximately $400,000 in fare and parking revenue,” Wiedefeld said in the letter. Wiedefeld said Metro “will take whatever measures [are] necessary to ensure safe operations” amid the shutdown.

“However, if ridership declines continue, in the short term, Metro could consider staffing or service adjustments, such as scaled back use of eight-car trains and extra trains to meet rush-hour demands.”

Wiedefeld added that the transit agency could be forced to seek additional funding from the District, Maryland and Virginia if it cannot find the money to continue operating the system in the longer term.

Meanwhile, the shutdown means federal grant money the agency is owed is not being disbursed, leaving the agency with $33 million in capital spending that has not been reimbursed. The transit agency will face a $50 million shortfall for January if reimbursements don’t come through, Wiedefeld said.

“If the federal shutdown continues for an extended period, Metro will be forced to either turn to its Line of Credit (LOC) to support the Capital program, incurring additional costs, or defer important state-of-good-repair projects, which could undermine our recent reliability gains,” Wiedefeld said in the letter.

In a joint statement Thursday night, Warner and Sens. Tim Kaine (D-Va.), Ben Cardin (D-Md.) and Chris Van Hollen (D-Md.) said the effects on Metro highlight the need for the shutdown to be ended swiftly.

“At a time when Metro already is undertaking substantial, disruptive projects to improve safety and reliability, President Trump’s shutdown is jeopardizing the health and stability of the entire Metro system,” the senators wrote. “This wasteful, destructive shutdown must come to an end.”

Metro declined to elaborate on the letter issued by Wiedefeld, saying the document “speaks for itself.”

“It would be premature for Metro to speculate on the long-term impacts,” Metro spokeswoman Sherri Ly said. Among the questions the agency declined to answer: How long can its $1.9 billion operating budget absorb steep operating losses before drastic action — such as a service cut — must be taken?

Metro declined last week to issue preliminary figures on how the shutdown was affecting ridership, citing a need to gather a more holistic portrait of the data. Internal ridership figures obtained by The Washington Post showed the transit agency was losing an estimated 50,000 rail trips per day compared with last year.

Metro also expressed concern that if the Federal Transit Administration is not up and operating soon, it could delay the certification of the Metrorail Safety Commission, the oversight panel set to assume safety oversight duties from the FTA. Without certification by the April deadline, the region risks losing $638 million in federal grant funding for fiscal 2019 — money that goes to jurisdictions and transit agencies across the District, Maryland and Virginia.

Still, Wiedefeld said, one element of the shutdown was not an issue — at least in the short term.

“Fortunately,” he wrote, “we do not anticipate any change to our credit rating.”