California Democratic leaders urged Congress on Friday not to eliminate programs that helped finance roughly two-thirds of the state's recent affordable housing supply in recent years.

"We cannot overstate the vital role these programs play in building and preserving affordable housing throughout the nation, but especially in California as we struggle with a housing crisis that is quickly metastasizing into a humanitarian and public health catastrophe," State Treasurer John Chiang wrote in a letter sent to Republican U.S. House Majority Leader Kevin McCarthy and other members.

California lacks an estimated 1.5 million affordable rental units. The Legislature this year passed a package of bills aimed at tackling the rising costs and crunched housing supply, including measures to cut regulations that stifle building and provide more money for construction.

Chiang's letter focused on a specific bond program that the GOP tax plan would eliminate. Without those bonds, developers can't apply for certain low-income housing tax credits that help them get financing to produce low-cost units. Together the programs created or preserved more than 20,000 units in 2016, with most set aside for households that earn less than 60 percent of an area median income, the letter said.

"At a time when our state has a deficit of more than 1.5 million affordable rental homes, we cannot afford to lose the single largest program helping to address this deficit," the letter said.

McCarthy spokesman Matt Sparks did not respond to a request for comment about the policy or the letter.

Assembly Speaker Anthony Rendon signed the letter, as did state Sen. Jim Beall, Assemblyman David Chiu, the executive directors of three housing organizations and the president of the State Building and Construction Trades Council.

Chiang is running for governor.