Former Obama administration official and San Antonio Mayor Julian Castro said his first goal as president would be passing "Medicare for All" legislation.

Castro, who has formed an exploratory committee on running for president, made the comment Thursday night when MSNBC host Chris Hayes pointedly asked what his first-choice bill would be if Democrats held Congress on the day he was inaugurated.

"You become president, you're sworn in, you have a Democratic House and Senate—the first bill, the first big domestic policy legislation you move is?" Hayes asked.

"Health care," Castro replied.

"Specifically?" Hayes followed up.

"I believe that we need Medicare for All," Castro said.

Medicare for All is the go-to term for expanding Medicare to a single-payer system for the entire United States, and it is estimated to cost $32.6 trillion. It’s most prominent proponent is self-declared socialist Sen. Bernie Sanders (I., Vt.), whose insurgent candidacy for the Democratic presidential nomination in 2016 rallied the American left before Hillary Clinton ultimately became the nominee.

However, Medicare for All has attracted prominent Democrats, including the bill’s co-sponsor in the Senate, Elizabeth Warren (Mass.). Notable new faces in the House of Representatives, such as Democratic socialists Alexandria Ocasio-Cortez (D., N.Y.) and Rashida Tlaib (D., Mich.), and Minnesota Democrat Ilhan Omar, have also backed Medicare for All.

Castro only spent a few seconds on the topic, but he told Hayes that it would be his first move after being sworn in because it will bring about universal coverage.

"I believe we need to recoup the millions of people who have already lost health insurance, and we need to do more than that and ensure that everybody has health insurance," Castro said.

Hayes thanked him for his clarity and then said every candidate needs to lead with health care in 2020.

"That's a good, clear answer. I want to get everyone running on this very question," Hayes said.

Castro has been one of the earliest Democrats to float running in 2020 to defeat President Donald Trump, and his brother, Rep. Joaquin Castro (D., Texas) told late-night host Stephen Colbert it is a sure thing his brother will run.