Sanders remains the only spender in Washington, North Dakota, Idaho, Georgia and Arizona, where both candidates will share a debate stage next Sunday. But in Florida, Biden has spent $3.4 million on ads, besting Sanders there by a $1.3 million margin. In Michigan, where both candidates will be campaigning this weekend and early next week, Biden has pulled even with Sanders and is now edging him by a $32,000 margin.

The millions of dollars in new ads is another show of strength from the resurgent Biden campaign, which now has more money to spend thanks to new donors and supporters since Biden romped in the South Carolina primary and then took over the Democratic delegate lead on Super Tuesday.

Sanders is countering the former vice president with ads hitting Biden's record. One slams past Biden comments on Social Security and Medicare, plays audio of the former vice president speaking from the Senate floor about the cuts he's made to those programs as well as veterans' benefits and "every single solitary thing in the government."

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"I not only tried it once, I tried it twice, I tried it a third time and I'll try it a fourth time," the Biden audio says juxtaposed against footage of Sanders promising to expand the same programs.

But Unite the Country, the super PAC that supports Biden, has matched the Biden campaign’s spending patterns in Michigan, Missouri and Mississippi, which will vote on Tuesday. The organization has spent close to $800,000 on ads in the states arguing that Biden is the strongest contender against Donald Trump in November’s general election.

The super PAC’s newest TV ad features soundbites from Biden’s victory speech in South Carolina: “Democrats want a nominee who’s a Democrat. An Obama-Biden Democrat.”

Biden also has new support from the party’s biggest spender, Mike Bloomberg, who shelled out $558 million on ads in his failed presidential bid. He endorsed Biden immediately after exiting the presidential race this week and Bloomberg's team has since announced plans to start a new outside group that will run pro-Democratic, anti-Trump ads to boost the Democratic presidential nominee and downballot candidates in battleground states.