WASHINGTON (REUTERS) - United States Democratic presidential candidate Elizabeth Warren won the backing on Monday of former rival Julian Castro, putting the only Latino candidate in a once historically diverse 2020 presidential field in her corner just weeks ahead of the first nominating contest.

Mr Castro last week became the latest Democratic candidate to end a White House bid. There are now 14 candidates vying to take on President Donald Trump in November 2020.

Mr Castro's endorsement of Ms Warren comes as the US senator from Massachusetts enters the final critical weeks before the nominating contests kick off on Feb 3 in Iowa. He could become a high-profile campaigner on Ms Warren's behalf as the contest moves on to states such as Nevada, the first with a sizable bloc of Hispanic voters.

"Elizabeth and I share a vision of America where everyone counts. An America where people⁠ - not the wealthy or well-connected - are put first. I'm proud to join her in the fight for big, structural change," Mr Castro, 45, wrote on Twitter.

He will make his debut with Ms Warren at an event in Brooklyn, New York on Tuesday.

When Mr Castro left the 2020 race, the former mayor of San Antonio, Texas, and former Housing Department chief during the Obama administration said in a video message to supporters that he had "determined that it simply isn't our time".

Though Ms Warren, 70, remains in the top tier of contenders, she trails former vice-president Joe Biden and fellow US Senator Bernie Sanders of Vermont in national opinion polls. Her fund-raising also dipped in the final quarter of 2019, when she brought in millions less than Mr Sanders and Mr Pete Buttigieg, the former mayor of South Bend, Indiana.

In a video Mr Castro posted along with his endorsement announcement, he and Ms Warren discuss the 2020 race over tea in her kitchen. "You did so many things in this campaign, and it continues to matter," she told him.