Julián Castro, the former US presidential candidate, has become the latest prominent Democrat to attack Joe Biden for his handling of the humanitarian crisis at the southern border, lamenting his use of a contested Trump-era policy and warning that the president’s “baffling” approach could tear the Democratic coalition apart.

In an interview with the Guardian, Castro said he was disappointed that the US president had remained silent about images of border patrol agents on horseback corralling Haitian migrants in Del Rio, Texas.

“Those images of Haitians being treated like animals demanded moral leadership from the president, it baffles me why he hasn’t said anything about it,” he said.

Castro also had strong words about the Biden administration’s ongoing use of Title 42, an obscure public health law that the Trump administration seized upon under the guise of the pandemic to clamp down on immigration across the Mexican border. Under Biden, the Department of Homeland Security has continued to invoke Title 42 as a means of quickly deporting more than 1,400 Haitians back to Haiti just this week.

Castro, who served under Barack Obama as housing and urban development secretary, said Title 42 mistreats desperate people by revoking their right, enshrined in US and international law, to claim asylum. It went against the advice of public health experts and was a direct contradiction of promises Biden made during the presidential campaign “to bring compassion and common sense to our immigration approach, instead of Trump’s cruelty”, Castro said.

He added: “This is a big mistake politically. If Biden doesn’t live up to those promises, he risks the collapse of the Democratic coalition that helped elect him.”

The situation at the border has rapidly developed into a full-blown humanitarian crisis, with up to 14,000 Haitians, many emanating from Central and South America, amassing in a makeshift camp in Del Rio earlier this week. US authorities have been returning hundreds of migrants each day on flights to Haiti.

The impact of the deportations has been aggravated by the multiple crises currently battering Haiti, including political collapse, gang violence and the aftermath of a major earthquake. “It amounts to cruelty to send people back to a place where there’s absolutely nothing for them,” Castro said.

As the border crisis intensifies, Biden is facing mounting criticism, even from within his own administration. On Thursday the US envoy to Haiti, Daniel Foote, resigned, expressing in an angry letter his dismay at the “inhumane” mass deportations.

Top Democrats have also spoken out. Chuck Schumer, the Senate majority leader, said on the Senate floor “we cannot continue these hateful and xenophobic Trump policies”.

The most troubling aspect for progressives of Biden’s apparent determination to stick with the rapid deportations of Haitians is that the use of Title 42 as an anti-immigration device was masterminded by Stephen Miller, one of the most controversial figures in the Trump administration. The former senior White House adviser is credited as being architect of many of Trump’s most brutal immigration policies, including the Muslim travel ban.

Miller also has a history of promoting white nationalism.

Castro told the Guardian that he had no doubt that Miller misused Title 42 “to further his white nationalism. It was a devious abuse of power to try and block people he didn’t like ever having a chance to live in the United States”.

Given its provenance, it was “more than unfortunate” that Miller’s ruse was continuing under the Biden administration, Castro added.

“Like Donald Trump, Stephen Miller had a dark heart towards asylum seekers and other migrants. That dark heart has no place in our approach.”

Castro, who now hosts the Our America podcast, said that he didn’t believe “by any stretch of the imagination that President Biden or his administration share the same horrible views as Miller. But as long as that policy is in place the effect is the same.”

In his Guardian interview, Castro added his voice to a growing chorus of complaints that aspects of Biden’s presidential leadership could be compared to Trump. The French foreign minister Jean-Yves Le Drian last week angrily noted that Biden’s unilateral submarine deal with Australia reminded him of “what Mr Trump used to do”.

Castro said “we can’t allow Trump’s policies to become the default policies of this administration. For whatever reason, this administration has treated immigration like a third-rail issue, afraid of it, mum on it too often, slow to make changes.”

During the 2020 presidential campaign, Castro called for a 21st-century Marshall plan for the so-called Northern Triangle countries of Central America. Repairing their economies would stem the flow of migrants to the US border, he argued.

Castro implored Biden to offer a similar positive vision now. “Work with the Northern Triangle countries and Haiti so that people can find safety and opportunity at home instead of having to seek it at the southern border.”