On Wednesday, Sen. Cory Booker (D-N.J.) became the first 2020 Democratic presidential candidate to propose penalizing cities that exacerbate the housing crisis.

Under Booker’s proposal, cities that refuse to allow dense forms of housing would become ineligible for federal transportation grants. The policy seeks to address the central conundrum of the housing crisis: America desperately needs more homes, but building them has become almost impossible in most of its major cities.

Booker’s proposal represents an ongoing paradigm shift in regulating America’s housing supply. Though the housing crisis has received unprecedented attention in the 2020 presidential campaign, until this week no candidate had proposed any measures to hold cities accountable for failing to address it.

“It’s a big step in the right direction,” said Michael Andersen, a housing researcher at the Sightline Institute, a sustainability think tank. “Federal transportation funding is important to every community. This would create an incentive for cities to embrace growth.”

The United States is in the midst of a severe housing shortage. According to the National Low Income Housing Coalition, America has just 37 affordable homes for every 100 extremely low-income renter households. In major cities, the numbers are even worse. Los Angeles, Las Vegas and Houston have enough affordable housing for fewer than 1 in 5 of their poorest renter households.

Cities have brought much of this upon themselves. Thanks to increasingly restrictive zoning codes and vocal not-in-my-backyard local activists, low-income housing and market-rate apartments have been rendered effectively illegal. This is especially true in wealthy neighborhoods. As a result, despite attracting hordes of new residents and thousands of new jobs, America’s municipalities are still building fewer homes than they were before the Great Recession.

So far, most of the action on reversing this dynamic has been at the state level. Earlier this year, California Gov. Gavin Newsom (D) proposed withholding gas tax funds from cities that fail to meet the state’s requirements to build new housing. In the state legislature, Sen. Scott Weiner (D) has twice attempted to pass laws that would force cities to permit apartment buildings near transit lines. Washington state is moving forward with legislation that would require cities across the state to allow backyard cottages and other “accessory dwelling units.”

“Housing is the biggest issue in a lot of Americans’ lives,” Andersen said, “but so far, the national political conversation has been slow to respond to that.”