Texas Democrat Julian Castro is expected to officially announce his candidacy for the Oval Office on Saturday, making him the first Hispanic in a jam-packed field of candidates seeking to challenge Donald Trump in the 2020 presidential election.

Mr Castro, 44, served as secretary of Housing and Urban Development under former President Barack Obama. Prior to that role, he was the mayor of San Antonio. Mr Castro is also the grandson of a Mexican immigrant and is likely to position himself in the election as a political outsider with liberal credentials.

The Texas Democrat announced forming an exploratory committee in December, and since then, he has started to assess the policy debates likely to become a driving force in the race to secure party nomination early next year.

“I’m not going to be a single-issue candidate,” Mr Castro told ABC last weekend. “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.”

So far, Mr Castro has endorsed “Medicare for all,” a proposal offering a national health care plan by allowing everyone and anyone to join the public health care system, a policy position expected to divide Democrats in the primaries. Centrist Democrats are likely to favour a less progressive approach to health care.

In the 2016 presidential election, Mr Castro was considered on the short list to be Hillary Clinton’s vice president pick. He is expected to leverage his Obama administration experience while simultaneously make the case that he is a political newcomer.

His identical twin brother is Democrat Texas Congressman Joaquin Castro, who sits on the House Intelligence Committee.