The former HUD secretary said he was "proud" to endorse the senator and join her fight for "big, structural change."
Julian Castro told MSNBC's Rachel Maddow the presidential nomination process involving caucuses in Iowa and New Hampshire "doesn't make any sense."
"I'm going to miss seeing my friend @JulianCastro on the trail," Senator Cory Booker tweeted.
Castro is the first presidential candidate to suspend a campaign since early December, when Senator Kamala Harris dropped out of the race.
In a follow-up tweet, Castro called for the breakup of ICE in order to "build a more humane immigration system."
He said his presidency would be careful not to be "overzealous" in regulating opioid prescriptions "in a way that hurts people who actually need medication."
Julian Castro says his brother will shave his head if people will donate $100K to his campaign within 24 hours, but some Twitter users say it is a desperate move.
Some of the candidates dominated the field, while others floundered on a crowded debate stage.
"By law, these migrants are supposed to be exempt from the Remain in Mexico policy—but [CBP] had decided to ignore their due process," 2020 Democratic presidential candidate Julián Castro said.
Co-hosted by GLAAD and The Advocate, multiple presidential candidates appeared, though Bernie Sanders was a notable no-show.
The trio only won by default; there were no huge, stand out moments other than Yang promising to give $1,000 a month to 10 families for the next year.
The GOP currently holds a three-seat majority in the Senate—and they have to defend more than 20 seats in the 2020 cycle.
"It was their first day of school. There are no words for this intentional cruelty," said Natalie Montelongo, the national political director for 2020 presidential candidate Julián Castro.
House Minority Whip Steve Scalise accused Rep. Joaquin Castro of "dangerous targeting" after he tweeted a list of donors to U.S. President Donald Trump.
The 2020 presidential hopeful lambasted the president's son for elevating a tweet that suggested Harris wasn't qualified to speak on behalf of black Americans.
"It makes you wonder. What are we hiding?" Democratic candidate Julian Castro said.
'The View' host pressed Julián Castro to be less "civilized" in the next debate, saying the liberal base wants to see him attack Donald Trump, Democrat frontrunners.
Castro also called on other Democratic candidates to promise to change the current immigration system.
"Or Russia," an audience member said as 2020 Democratic candidate Castro shared his hopes about winning the White House and sending off the Trumps.
The new campaign policy was made directly in response to a recent incident in Florida, where a officer was caught on video brutally beating a 15-year-old black male.
"The next president must start by reversing the cruel policies of the Trump administration ," Democratic presidential candidate Julian Castro said in an interview with NPR.
Senator Cory Booker from New Jersey on Friday tweeted that "we can do this" in reference to the Green New Deal, while Senator Kirsten Gillibrand from New York announced last week that she would be "co-sponsoring" the resolution.
The Democratic and Republican leaders in the 2016 race for the White House are unusually unpopular.
"I am going to look really hard at him for anything, because that's how good he is," Clinton said.