Democratic 2020 presidential candidate Julian Castro on Monday called on Congress not to shield itself from large swathes of the Freedom of Information Act.

Castro, the former Obama administration housing secretary and mayor of San Antonio, Texas, made the comments at the We the People Membership Summit, an event held in Washington, D.C., for liberal activists to discuss electoral and economic reforms with eight White House contenders.

"We have to make Congress subject to the Freedom of Information Act,” he said. “We need to shine a light on what happens in Congress."

Castro reiterated his push to abolish the Electoral College because of its track record in not reflecting the "will of the people," a reference to the popular votes in the 2000 and 2016 presidential elections, and encouraging White House hopefuls to focus their time and energy on a handful of battleground states.

The country should also move toward a commission-based form of congressional electorate redistricting, he said.

Monday's summit was organized by the Center for Popular Democracy Action, Communications Workers of America, Planned Parenthood Action Fund, Service Employees International Union (SEIU), 32BJ SEIU, and the Sierra Club. Sens. Cory Booker of New Jersey, Kirsten Gillibrand of New York, Amy Klobuchar of Minnesota, Bernie Sanders of Vermont, and Elizabeth Warren of Massachusetts, as well as former Rep. Beto O'Rourke of Texas and Washington state Gov. Jay Inslee, are additionally scheduled to address the estimated more than 1,000 attendees.

[Read more: Julian Castro becomes the latest 2020 Democrat to support tearing down border wall]