The event is being sponsored by Bredesen's joint fundraising committee, the Bredesen Victory Fund, which supports the campaign and the Tennessee Democratic Party. The funds at the lunch are expected to be divided between Bredesen's campaign and the states party's coalition, the invite says.

Biden is slated to be the "special guest" at an afternoon fundraiser described as "coffee and conversation" with the former VP and longtime senator, according to an invitation provided to CNBC through sources familiar with the event. The campaign is charging as much as $10,000 for each attendee followed by a $5,000 fee and a base entrance charge of $2,500 according to the invitation.

The former Tennessee governor's cash flow could get a boost at a Tuesday fundraiser, where attendees at a private home in Nashville will pay exorbitant prices to get a glimpse at Biden – who is touted as a strong contender for the Democratic presidential nomination in 2020.

Early polls give Democratic former Gov. Phil Bredesen an edge over GOP Rep. Marsha Blackburn in the race to fill Republican Bob Corker 's Senate seat, but she has the clear edge when it comes to money.

"During a 24-day sprint through the holidays after announcing December 7, the Bredesen campaign raised more than $500,000 – with no fundraising apparatus in place, which is more than some incumbent U.S. senators raised during the entire fourth quarter," a campaign spokeswoman said in a statement provided to CNBC. "It is clear that Governor Bredesen's message of working together to get things done is resonating with Tennesseans, but our campaign is taking nothing for granted and will work to earn every vote," she added.

The campaign needs more cash if it wants to compete for a seat that has been held by Republican Corker since 2007.

The joint fundraising committee has yet to bring in a single donor, and the campaign has raised $509,000, with $493,000 on hand, according to Federal Election Commission filings reviewed by CNBC.

That may not be enough for Bredesen to have a chance against Blackburn, who is the GOP frontrunner to hold onto the seat, outraising the rest of the field. Corker, who is retiring but did briefly reconsider jumping into the race, tweeted Monday that he would be contributing to Blackburn.

CORKER TWEET

She has raked in $3 million and has received backing of a powerful political network financially supported by Charles and David Koch, two of the most influential executives in American politics, financial records show. The Club for Growth political action committee, the sister organization of the network's nonprofit, has been one of Blackburn's largest financial supporters throughout the year, totaling just over $70,000 in contributions.

Even the donors that Bredesen has been able to bring into the fray are aware of the difficulties he faces in the traditionally Republican state. They acknowledge that the campaign is going to have to step up its fundraising game in order to flip the Senate seat in the Democrats' favor.

"I do think he has to raise a lot of money," Keith Simmons, former managing partner at the law firm Bass, Perry & Sims and top donor to the Bredesen campaign, told CNBC in an interview. "It's a red state so whether it's against Marsha Blackburn or someone else, he needs to raise a ton of money to have a fighting chance."

Simmons has contributed $5,400 to the campaign so far.

Blackburn herself might have some work to do, according to the few polls on the race. According to a poll last week from Middle Tennessee State University, 45 percent of 600 registered voters said they would chose Bredesen over Blackburn, who scored 35 percent.

A Blackburn campaign spokeswoman did not return requests for comment.

For Biden, the Tennessee appearance looks like it will be just one of many stops this year to help Democrats win in this year's midterm elections.

"He's going to do to help Democrats get elected in 2018," Bill Russo, Biden's spokesman, told CNBC. "He starting making some house endorsements and he's been doing some Senate fundraisers as well."

As for the bigger question about whether the former vice president will run in 2020, Russo was a lot less definitive.

"He hasn't made a decision yet," Russo said.