A subsidy designed to encourage federal workers to take public transportation will drop to $130 from $245 starting Jan. 1 — a shift that has officials at Metro concerned about their bottom line since many of their weekday customers are federal workers.

The decrease also will impact commuters whose employers allow them to set aside pretax dollars to pay their commuting costs.

The reduction could be bad news for Metro, which along with public transportation advocacy groups, has long argued that subsidies and pretax transit benefits help encourage commuters to leave their cars at home and take buses and trains.

“This potentially has a devastating effect on ridership as people decide to take other modes of transportation to get to work, particularly if the parking subsidy continues to go up while the transit subsidy continues to go down,” said Tom Downs, chairman of Metro’s board of directors, during a recent board meeting.

Downs urged commuters to contact their representatives in Congress.

The amount federal workers in the National Capitol region receive and the pretax amount other commuters could set aside has varied over the years. In 2011, the federal commuter benefit was capped at $230. In 2012, that amount dropped to $130. But this year, as part of the “fiscal cliff” negotiations the benefit increased to $245 and included a provision that made it retroactive to 2012. But the agreement on that benefit level was only good through the end of this year.

This year, there was a push to make the $245 amount permanent, but it has stalled in Congress.

Metro officials have blamed revenue losses in part on cuts to the federal transit benefit subsidy for federal workers. Forty percent of Metro’s peak period travelers are federal employees whose rides are subsidized.