WASHINGTON ? Democratic presidential candidates are adopting a more combative tone on the campaign trail ahead of next week’s debates in Detroit, laying out their disputes from afar as desperation grows for some of those fighting for survival.

Former Colorado Gov. John Hickenlooper has warned that the party could risk losing in 2020 if its eventual nominee can be easily tagged with the “socialist” label – a reference to more progressive candidates in the race such as Vermont Sen. Bernie Sanders, with whom Hickenlooper will share the stage next Tuesday.

Sen. Michael Bennet of Colorado said this month that President Trump was more likely to win battleground states such as his if Sanders wins the Democratic nomination.

New Jersey Sen. Cory Booker this week called former Vice President Joe Biden the “proud architect of a failed system” on criminal justice, after Biden released a criminal-justice proposal that repudiates aspects of the 1994 criminal-justice overhaul.

Biden countered Wednesday, saying that Booker, while he was Newark’s mayor, oversaw a police department that engaged in “stop-and-frisk” tactics and needed federal intervention.

“If he wants to go back and talk about records, I’m happy to do that, but I’d rather talk about the future,” Biden told reporters in Dearborn, Mich., after addressing the NAACP convention.

The jabs between Biden and Booker, who will stand next to each other at Wednesday’s debate along with Bennet, underscore tensions within the 2020 campaign ahead of an expected winnowing process this summer and fall.

Booker is one of several candidates who have been polling in the low single-digits and are looking to ignite their campaigns.

New York Mayor Bill de Blasio, speaking on CNN this week, portrayed Biden as part of the status quo and someone unlikely to bring about the dramatic change in federal policy desired by the party’s base.

In the late-June debates, former Housing and Urban Development Secretary Julián Castro of Texas criticized former Texas congressman Beto O’Rourke for what he described as views on immigration policy that are too restrictive.

Twenty candidates will appear on the debate stage over two nights in Detroit – 10 each night – in what could be the last dose of public attention for many of the candidates struggling to gain traction.

The next debate, set for Houston in September, could include less than half of the current field because it requires higher polling and donor thresholds for participation. Both Hickenlooper and Bennet would have trouble qualifying as things stand now.

The Detroit debate could serve as an opportunity for Biden to rebound after a confrontation during the first debate in late June with California Sen. Kamala Harris. She criticized Biden over his opposition to mandatory school busing in the 1970s and comments he had made recently about cooperating with segregationist senators decades ago.

Harris has seen her standing in the polls rise since she confronted Biden, who holds a narrow advantage over his rivals less than six months before the Iowa caucuses start the nomination voting.

“I’m not going to be as polite this time,” Biden told donors at a fundraiser in Detroit late Wednesday in reference to his debate strategy and Harris. “This is the same person who asked me to come to California and nominate her in her convention.”

One of the debates will also feature the first face-off between Sanders and Massachusetts Sen. Elizabeth Warren, who are vying for support from the party’s most liberal voters.

In a CNN interview Tuesday, Sanders offered a clipped response when he was asked what he admires about Warren. He said she “is a friend of mine and I admire the fact that we have worked together over the years on a number of issues.”

Asked for specifics, Sanders replied, “We’ve worked together on a number of issues and she is a very good senator.”

Warren was positive when asked recently about sharing a debate stage next week with Sanders. “I am delighted,” she said. “Bernie and I have been friends for a long, long time.”

Booker’s recent criticism of the former vice president referred to Biden’s role in shepherding through Congress the Violent Crime Control and Law Enforcement Act of 1994, which most Democrats supported at the time but which critics have faulted for fueling high incarceration rates among black men.

Biden’s new proposal would seek an end to mandatory minimum sentences and pump billions into the juvenile justice system.

Kate Bedingfield, a top campaign adviser to Biden, said in a statement that Booker oversaw “a police department that was such a civil-rights nightmare that the U.S. Department of Justice intervened.”

Booker campaign manager Addisu Demissiein responded in a tweet that echoed a Biden campaign statement. “For decades, Joe Biden has been working on criminal-justice reform,” he wrote. “That’s the problem.”

Booker was elected mayor of Newark in 2006 and served until he was voted into the Senate in a special election in October 2013.

The Justice Department, in a 2014 report, said it opened an investigation of the Newark Police Department in May 2011 after receiving serious allegations of civil-rights violations that included subjecting residents to “excessive force, unwarranted stops and arrests, and discriminatory police actions.”

That same report also praised the city for “acknowledging the community’s concerns and cooperating with the investigation” and for reaching an agreement with the federal government to “remedy the problems identified by this investigation.”

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