They were born a minute apart, Julián at 2.40am and Joaquin at 2.41am, and for the next 20 years were virtually inseparable.

They went to school together, played tennis as a doubles team, shared classes at Stanford and Harvard law. They look and speak so identically that local media in their hometown of San Antonio, Texas, have posted guides on how to tell them apart.

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Now the Castro twins, 44, find themselves united once again, this time at the frontlines of the increasingly bitter battle over Trump’s draconian race-baiting at the US-Mexico border.

Though they travel on different political paths – Julián as contender for the 2020 Democratic presidential nomination, Joaquin as congressman and chair of the Congressional Hispanic caucus – they are confronting Trump with one voice.

Julián, the former mayor of San Antonio and a housing secretary in the Obama administration, fired the first salvo. He sent a jolt through the audience at the first televised Democratic presidential debate in Miami last month when he invoked the image of a father and daughter drowned in the Rio Grande while trying to swim to safety in America, saying: “It should piss us all off.”

Five days later, Joaquin ignored the requests of guards in the Clint border detention center in Texas by filming the inhumane conditions in which Central American immigrants were being held. The images posted on Twitter reverberated around the world.

Facebook Twitter Pinterest Julián Castro: ‘You can’t run away from him, you can’t squirm, you have to take the bully head on.’ Photograph: Ethan Miller/Getty Images

Both acts were strikingly defiant, almost aggressive, in their challenge to Trump’s anti-immigrant and anti-Latino border policy. In interviews with the brothers, the Guardian asked them whether their forceful stance was a conscious tactic in confronting the increasingly virulent US president.

“The times absolutely call for a tough approach,” Julián Castro said. “You have to face a bully – and that is what Trump is – with a strong response. You can’t run away from him, you can’t squirm, you have to take the bully head on.”

Joaquin Castro agreed. “You must respond with the proper force and strength. We are not people, my brother and I, who gratuitously look for fights but this is the time to stand up, right now.”

The twins’ robust stand on immigration comes as Trump is dramatically ramping up his rhetoric on the border. He has twice threatened mass raids on undocumented migrants that have yet failed to materialize but have spread fear among Latino communities across the country.

Trump has also vowed to cancel all rights to claim asylum for migrants turning up at the border having passed through a third country – a move widely believed to be unconstitutional that would hit all Central American asylum seekers.

Again, the brothers speak in one voice in denouncing Trump’s increasingly shrill postures. “Racial and ethnic resentment is like psoriasis,” said the congressman. “It recedes and you think that it’s gone and then somebody comes along and scratches it. That’s Donald Trump’s playbook – he’s an agitator.”

Facebook Twitter Pinterest Joaquin Castro filmed the inhumane conditions inside the Clint facility in Texas, where migrants were being held. Photograph: Paul Marotta/Getty Images

Joaquin points to the president’s recent baiting of the four progressive Democratic Congresswomen who he told to “go back” to the countries they came from despite the fact that three were born in the US and all four are US citizens. Trump’s provocations culminated with a crowd of his supporters at a rally in North Carolina last week chanting “Send her back!” in reference to the congresswoman from Minnesota, Ilhan Omar.

“The president is engaging in racial priming – he’s priming the pump of racial division,” Julián told the Guardian. “That’s how he got elected in 2016 and that’s how he thinks he’ll get elected in 2020. That’s what he’s up to, and it’s incumbent on all of us who oppose that to speak out against it and present a strong alternative plan.”

So far, the Castros’ strident interventions are having a positive impact for them. Julián’s outspoken performance in the Democratic debate, combined with his framing of the most detailed immigration plan of any Democratic presidential candidate, has seen his fortunes rise.

The New York Times lauded him winner of the first night of the debates. He has also passed an important landmark to qualify to be on stage at the September TV debates by attracting more than 130,000 unique donors, while one poll by ABC/Washington Post put him in fifth place with 4% of the Democratic vote.

“If you’re not strong and clear in what you say you are not going to rise above the noise. People are responding to my vision for how we can fix our broken immigration system,” he said.

Joaquin has also seen his forthright stance lead to results. He points out that after he refused to abide by the guards’ edict to stop filming inside Clint, other members of Congress have followed suit and done their own defiant recording inside detention centers.

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That the Castro twins should find themselves in such a similar spot aged 44is not so extraordinary given their close trajectory. The first time they spent any time apart was when Julián was about 20 and went to work as an intern in the White House.

What is perhaps surprising is that two politicians who have a reputation for being carefully spoken, even diffident, have now come out guns blazing. But Joaquin rejects their characterization as the good boys of Democratic politics.

He said he had started to think about his mother, Rosie, a Mexican American activist and proud Chicana who was prominent in Latino civil rights struggles in the 60s and 70s.

When the twins were children she would be taking part in pickets and strikes, organising walks outs, even on occasion getting arrested.

When the brothers entered politics in their 20s – Julián as mayor of San Antonio, Joaquin as a Texas legislator – they took the elected route and worked from within.

“But now it’s all changing. We are being asked to stand up, just like my mother did.”