Washington (AFP) - Twin brothers Joaquin and Julian Castro are shaking up US presidential politics by forcing attention on the desperate situation of migrants, one as a congressman and the other the sole Latino seeking the Democratic White House nomination.

With Democrats generally backfooted by President Donald Trump's tough crackdown on undocumented migrants, the Castros stole the spotlight in recent days by highlighting the mistreatment of border-crossers and offering concrete ideas to face the crisis in a more humane way.

Last week Julian, the former mayor of San Antonio, Texas, grabbed center-stage in the first Democratic presidential debate with specific proposals for action on illegal immigration.

He took direct aim at fellow Texas Democrat, Beto O'Rourke, for equivocating on migrant policy, but his stance set him apart from the entire Democratic field.

Then Joaquin, the Democratic representative in Congress for San Antonio and Julian's campaign manager, rocked the news cycle Sunday by releasing clandestine videos of migrant adults and children crammed into holding cells near the US-Texas border, where, he said, many go for weeks without showers or needed medication.

"The American people must see what is being carried out in their name," Joaquin Castro said, calling Trump's approach "morally bankrupt."

It has appeared to give Julian Castro's campaign a boost.

According to the Washington Post-ABC News poll Tuesday, he has passed O'Rourke and is tied with another young city mayor, Pete Buttigieg, with four percent.

- Inseparable twins -

At 44, the Castros are leading a new generation of Latino politicians increasingly crucial to the Democratic Party's electoral future.

Identical twins with near identical career paths, they are the grandsons of a woman who moved to the United States as an orphan a century ago, and their mother, Rosie Castro, is a longtime Texas political activist.

They grew up in San Antonio, and attended elite Stanford University, studying political science. Inseparable, they then together went to Harvard Law School.

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After graduating they both launched political careers, Julian on the San Antonio city council and Joaquin in the Texas statehouse.

When Julian won the San Antonio mayor race in 2009, he became the youngest mayor in any of the country's 50 largest cities, and his star rose quickly.

In 2012, President Barack Obama chose him to deliver the opening night keynote speech at the Democratic Party convention -- the same role that lifted Obama eight years earlier.

Julian told the crowd in Charlotte, North Carolina that he had lived the American Dream, coming from impoverished immigrant grandparents who warked hard to make ends meet.

"In the end, the American dream is not a sprint, or even a marathon, but a relay. Our families don't always cross the finish line in the span of one generation."

He was quickly dubbed the "Latino Obama," and in 2014 joined Obama's cabinet as secretary of Housing and Urban Development, earning more national exposure.

Joaquin, meanwhile, won a seat in Congress in 2012, where he has become a leader in the important Hispanic Caucus and a senior member of the House Foreign Affairs Committee.

- 'Compassion instead of cruelty' -

While either twin could have taken a stab at presidential politics, they appear to have decided on Julian -- though for the public, it's hard to tell one from the other.

In the extremely crowded field for the 2020 Democratic nod, their push on immigration has helped Julian stand out

He says the country must first decriminalize illegal border crossing, and then push more aid to Central American countries where migrants originate.

"That law that criminalizes them is the main law that allows this Trump administration to incarcerate the parents and then separate them from their little children," he told CBS News on Tuesday.

"This is not open borders. It does mean that we deal with people with common sense and with compassion, instead of with cruelty."

Before the debate, Julian was ranked eighth with less than two percent support from Democrats.

But in the "Survivor"-like competition, his surprise attack on the vulnerable O'Rourke, running sixth in the field, was seen as a shrewd tactic, producing a much-needed surge in the polls.