WASHINGTON (Reuters) – Texas Democrat Julian Castro, a former San Antonio mayor who went on to be the top U.S. housing official, was expected to formally announce his White House bid on Saturday, the first Hispanic in what looks to be a crowded field of candidates vying to challenge President Donald Trump in the November 2020 election.

Castro, 44, the grandson of a Mexican immigrant, served as secretary of Housing and Urban Development under former President Barack Obama and has long been viewed as a rising star in the party. He will seek to position himself as a political outsider with liberal credentials.

Since announcing formation of an exploratory committee in December, Castro has begun to stake out positions on policy debates that will dominate the nominating contests that kick off early next year.

“I’m not going to be a single-issue candidate,” he told ABC’s “This Week” last Sunday. “My vision for the country’s future is that we aim in the 21st century to be the smartest, the healthiest, the most fair and the most prosperous country.”

Castro has endorsed the “Medicare for all” proposal, which would in effect create a national health care plan by allowing anyone to join the public health care system. That policy point is likely to divide Democrats in the primary, with more moderate candidates favoring a less drastic approach.

Castro, whose grandmother was born in Mexico, has sought to use his family’s personal story to criticize Trump’s border policies.

Castro, who will make his announcement in San Antonio, would be the second candidate to formally launch a campaign. Former U.S. Representative John Delaney has been running for more than a year, and U.S. Senator Elizabeth Warren of Massachusetts has formed an exploratory committee and has begun holding campaign events in Iowa and New Hampshire, the states with the earliest contests.

More than a dozen potential Democratic candidates are exploring a possible run for president in 2020. Moderates and progressives in the party have been debating about how to best challenge Trump, the likely Republican nominee.

Some Democrats believe an establishment figure who can appeal to centrist voters is the way to win back the White House. Others contend a fresh face is needed to energize the party’s increasingly left-leaning base.

Castro, who was considered on the short list to be Hillary Clinton’s running mate in the 2016 election, will try to leverage his Obama administration experience while making the case he is still a political newcomer.

Castro’s identical twin brother, Joaquin Castro, is a member of the U.S. House of Representatives from Texas. Joaquin Castro’s position on the House Intelligence Committee has made him a frequent public critic of the president.