Sen. Bernie Sanders, I-Vt., on Saturday expressed doubt that Democratic presidential primary rival Joe Biden can “generate enthusiasm” due to the support he is getting from dozens of billionaires -- while promising to back the former vice president if he is the nominee.

“The American people are disgusted with billionaires buying elections. We believe in democracy, one person one vote, not billionaires spending hundreds of millions of dollars ” Sanders told supporters in Dearborn, Mich. “I just don't think that Joe Biden can generate enthusiasm when you got 60 billionaires contributing to his campaign,” he said to boos.

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He went on to accuse Biden's backers of running “ugly” ads against him. The two men have trained their sights on each other after most other candidates have pulled out of the race -- with only Rep. Tulsi Gabbard, D-Hawaii, as the third remaining candidate.

“When you've got, Joe Biden has a Super PAC with billionaires putting money into that Super PAC and right now our campaign has taken on a Super PAC running ugly negative ads against us all over the country,” Sanders said.

After Biden scored major wins on Super Tuesday, restoring his status as frontrunner, Sanders has sought to draw a bigger distinction between them -- casting doubt on Biden’s ability to fight for the American people.

“At the end of the day, people understand that if you're taking lots of money from billionaires, you’re not going to be there standing up for the working class and the middle class of this country,” he said.

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However, he also said he would support Biden if he got the nod from the Democratic Party, something that Biden has also said about Sanders.

"Because we know how important it is to beat President Trump," Sanders said.

The remarks indicate the race for the nomination is intensifying. The self-described democratic socialist has targeted Biden since before Super Tuesday, but that has increased since Biden swept 10 of the 14 Super Tuesday states.

This week, Sanders accused Biden of supporting banning openly gay Americans from serving in the military when he was in the Senate. Biden campaign national press secretary Jamal Brown responded by saying Sanders was “lying.”

“[Biden] is on record saying he didn't have an issue with gays & lesbians in the military,” he said.

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With 125 pledged delegates at stake, Michigan is the biggest prize among the six states holding primaries on Tuesday, March 10. Missouri, Mississippi, Washington State, Idaho and North Dakota also hold contests.

Sanders told reporters on Thursday: "Michigan is where we'll spend a bit of our time."

Fox News' Paul Steinhauser contributed to this report.