Liberal US senator Bernie Sanders has praised Donald Trump’s drive to clampdown on illegal immigration and fentanyl in an admission that Joe Biden’s administration failed to adequately tackle the issues.

Sanders, the Vermont independent who caucuses with Democrats, made the surprising concession in an interview with ABC’s Jonathan Karl on Sunday that was otherwise noteworthy for his threat to end it prematurely after the host asked him about the political future of progressive congresswoman Alexandria Ocasio-Cortez.

Ocasio-Cortez and Sanders have recently addressed mass audiences in a joint “fight oligarchy” tour that saw them try to tap into popular opposition to Trump’s policies in the first two months of the Republican’s second presidency.

Asked if there was anything Trump had done right, Sanders replied: “Yeah. I think cracking down on fentanyl, making sure our borders are stronger.”

He went on: “Nobody thinks illegal immigration is appropriate, and I happen to think we need comprehensive immigration reform, but I don’t think it’s appropriate for people to be coming across the border illegally, so we’ve got to work now on comprehensive immigration reform.”

Challenged by Karl to explain why illegal immigration had “exploded under Biden” but “nothing was really done until his last year in office”, Sanders replied: “Yes. Should have done better. No argument.”

Sanders implicitly criticised Trump’s attempt to carry out mass deportations of undocumented immigrants – a process that has already begun with flights of deported immigrants to Panama, El Salvador and Colombia. He warned that a mass exodus of undocumented workers could devastate the US economy.

“The idea that Trump has … he says he wants to deport 20 million people who are in this country who are undocumented,” Sanders said. “Well, you do that, you destroy the country, because I got news for you – Trump’s billionaire friends are not going to pick the crops in California that feed us. They’re not going to work in meat packing houses. That’s what undocumented people are doing.”

He said the situation called for “a variety of programs”, including “guest worker” schemes. But he said it should be ultimately tackled through “comprehensive immigration reform”.

Sanders’s guarded endorsement of Trump’s stance represents a significant hardening of his past position. His 2020 presidential campaign platform called for the breaking up of the Immigration and Customs Enforcement (Ice) as well as US Customs and Border Protection (CBP) agencies as well as pausing deportations pending an audit.

The platform also called for “a humane policy for those seeking asylum” and extolled the image of the US as “a nation of immigrants”.

“We now have a president who is a racist, a xenophobe, and a demagogue,” the platform said, referring to Trump, who lost re-election that year to Democratic challenger Biden before retaking the presidency in November. “The Trump administration’s treatment of immigration exclusively as a criminal and national security matter is inhumane, impractical, and must end.”

Sunday’s interview was also marked by a testy response from Sanders after Karl asked him if he would like to see Ocasio-Cortez, a US House member from New York, run for the Senate.

Sanders called the question “nonsense” and stood up to end the interview after saying: “I don’t want to talk about inside-the-Beltway stuff.”

He relented and completed the interview after Karl told him he had one more question.