(CNN) Democratic presidential candidates went after former Vice President Joe Biden's immigration record on the debate stage Wednesday night, calling on him to respond to the millions of deportations that occurred under President Barack Obama.
Biden refrained from disclosing his conversations with Obama but defended the former president, saying Obama had "moved to fundamentally change the system."
Obama's immigration policies drew criticism from some immigration advocates and lawmakers, and it had been expected that Biden would be called to answer to them. The former vice president indicated that he didn't intend to buckle on those policies, instead raising the key issues Obama had pressed forward on, like addressing the root causes of migration from Central American countries and providing a pathway to citizenship for undocumented immigrants.
But it was Obama's deportation record, which earned him the moniker "deporter in chief," that resulted in a testy exchange between New York Mayor Bill de Blasio and Biden.
"You were vice president of the United States. I didn't hear whether you tried to stop them or not using your power, your influence, in the White House. Did you think it was a good idea or did you think it was something that needed to be stopped?" de Blasio said referring to deportations.
Biden responded, "The President came along, and he's the guy that came up with the idea -- first time ever -- of dealing with the Dreamers," referring to undocumented immigrants who came to the US as children.
Biden then listed a series of items the former administration accomplished, including attempts at comprehensive immigration revisions and investment in Central America.
"To compare him to Donald Trump, I think, is absolutely bizarre," Biden said.
Obama created a program, Deferred Action for Childhood Arrivals, shielding undocumented immigrants who had arrived in the US as children from deportation and allowing them to legally work in the country. The program, however, was not enshrined into law and has left recipients in limbo, as the Supreme Court prepares to review President Donald Trump's decision to terminate the program.
Obama also tried to usher forward comprehensive immigration restructuring. Sen. Michael Bennet of Colorado, who was also on the debate stage with Biden on Wednesday, worked on an immigration bill in 2013 that included a pathway to citizenship for 11 million people and $46 billion on border security.
While the bill passed the Senate, then-Speaker John Boehner did not take it up in the Republican-controlled House, and the legislation died at the end of 2014.
In 2016, the Obama administration committed $750 million in investment in the so-called Northern Triangle -- the Central American countries of El Salvador, Guatemala and Honduras -- for development assistance, law enforcement support, and other training and aid, following a Biden-led effort in the region.
But this spring, Trump moved to undo those investment efforts, directing the State Department to cut aid to the Northern Triangle in response to the migrant caravans that arrived on the US southern border.
"I already proposed and passed $750 million for Guatemala, El Salvador and Honduras to be able to change the circumstance why people fled in the first place," Biden said Wednesday in defending his position to not decriminalize border crossings as Democratic candidate Julian Castro has proposed.
Biden argued that there are people all around the world waiting "in line" to come to the US.
"Mr. Vice President, it looks like one of us has learned the lessons of the past and one of us hasn't," Castro quipped. Castro served as the secretary of housing and urban development under Obama.