CARSON CITY, Nev. (AP) — With the Nevada caucuses less than a week away, Democratic presidential candidates campaigning were fixated on a rival who wasn't contesting the state.

Bernie Sanders, Joe Biden, Amy Klobuchar, Elizabeth Warren and Pete Buttigieg all went after billionaire Mike Bloomberg and made clear they were eager to take him on in a debate.

“He thinks he can buy this election,” Sanders told a Carson City rally Sunday. “Well, I’ve got news for Mr. Bloomberg — the American people are sick and tired of billionaires buying elections!”

Bloomberg hit back Monday with a video mashup posted to Twitter of aggressive and threatening comments made by people who appear to be Sanders supporters, juxtaposed with Sanders calling for “civil discourse.”

“We need to unite to defeat Trump in November," the former New York mayor tweeted. “This type of ‘energy’ is not going to get us there.”

Their attacks are a sign of how seriously the field is starting to take Bloomberg as he gains in the race and is on the cusp of qualifying for Wednesday's Democratic debate in Las Vegas. Bloomberg has bypassed the traditional early voting states including Nevada, focusing instead on the 14 states that vote in the Super Tuesday primary on March 3. He has spent more than $417 million of his own multibillion-dollar fortune on advertising nationwide, an unprecedented sum for any candidate in a primary.

The focus on Bloomberg comes with many establishment-aligned Democrats anxious about the early strength of Sanders, who won last week's New Hampshire primary and essentially tied for first place in Iowa with Pete Buttigieg, the former mayor of South Bend, Indiana. Sanders is hoping to notch a victory in Nevada on Saturday as moderates struggle to unite behind a candidate who could serve as a counter to the Vermont senator, who has long identified as a democratic socialist.

The hundreds of millions of dollars that Bloomberg has pumped into the Super Tuesday states has only heightened the sense of uncertainty surrounding the Democratic race.

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At Sanders’ rally, the crowded cheered as the Vermont senator joked that Bloomberg is “struggling, he’s down to his last $60 billion" and derided him for skipping the early primary states.

It marked an escalation of the salvo Sanders launched Saturday against the former mayor, when he ticked off conservative positions Bloomberg has taken in the past, including opposing a minimum wage hike and a number of Barack Obama's policies while president. On Saturday, Sanders suggested the former mayor's past conservatism and controversial comments make him a weak candidate against President Donald Trump, charging that Bloomberg, “with all his money, will not create the kind of excitement and energy we need” to beat Trump.

And on Sunday, he was joined by the current mayor of New York, Bill de Blasio, who endorsed Sanders days ago. De Blasio introduced Sanders with an attack of his own on his predecessor, telling the crowd, “I’m sorry to report to you the chief proponent of stop and frisk is now running for president.”

Klobuchar, speaking on CBS’ “Face the Nation,” accused Bloomberg of avoiding scrutiny by blanketing the airwaves and sidestepping debates or tough televised interviews.

“I think he cannot hide behind the airwaves and the money," she said. “I think he has to come on the shows. And I personally think he should be on the debate stage.”

Klobuchar said she's raised $12 million since her better-than-expected finish in third place in New Hampshire. She's maintained her campaign through a series of strong debate performances and argued that Bloomberg being on stage with his rivals would level the playing field.

“I’m never going to beat him on the airwaves, but I can beat him on the debate stage,” she said.

Biden, speaking on NBC's “Meet the Press,” suggested that Bloomberg will face increased scrutiny as the race continues, pointing to his record on issues relating to race. He said: “$60 billion can buy you a lot of advertising, but it can't erase your record."

Biden knocked Bloomberg’s past support of stop-and-frisk policing policies and his comments suggesting that cracking down on racist mortgage lending practices, known as “redlining,” contributed to the financial crisis. Biden also criticized him for failing to endorse Obama for president in 2008. Bloomberg has released ads that tie him closely to Obama on issues like gun control and climate change.

When asked on MSNBC whether Bloomberg shares the values of the Democratic Party, Warren also went after the former mayor over his comments on redlining, declaring that “anyone who is out there trying to blame African Americans for the financial crash of 2008 ... is not someone who should be representing our party.”

Even as the front-running candidates kept one eye on their Super Tuesday showdown with Bloomberg, they focused on the more immediate task of winning over minority voters, who are expected to be pivotal in Nevada and South Carolina.

Biden reminded older parishioners at the First African Methodist Episcopal Church in North Las Vegas of 1960s television footage of black protesters in Birmingham, Alabama, being attacked by police dogs and sprayed with fire hoses on the orders of city official Bull Connor.

Biden said today's racists are not “Bull Connors, not out in overalls. They're wearing fine suits, and they're living in the White House.”

The former vice president is relying on his strength among black voters and an explicit appeal to Latinos and other minorities to deliver him a strong showing in the coming contests after posting disappointing finishes in Iowa and New Hampshire, which both feature electorates that are whiter on average than the national population.

Biden has been hammering home the need for any Democratic candidate to appeal to voters of color. On Sunday, he told black lawmakers and other political figures at the Nevada Black Legislative Caucus's Black History Month observance that “the black community has in its power to determine who the next president of the United States is going to be."

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Jaffe reported from Washington. Associated Press writers Jonathan Cooper and Bill Barrow in Las Vegas contributed to this report.