Des Moines, Iowa (CNN) They've hired staff, opened offices, signed up volunteers, canvassed across the state, and committed thousands of Iowans to caucus on February 1. And for Democratic presidential hopefuls, the first test of all of that grassroots support is Saturday in Des Moines at the Iowa Democratic Party's Jefferson-Jackson Dinner.

The dinner is a big pep rally dinner for Iowa Democrats, and this year, it's an important stop for the three remaining Democratic candidates looking to seal the deal with potential supporters 99 days ahead of the caucuses.

The pressure is especially on for Bernie Sanders, who, in a Des Moines Register/Bloomberg poll out this week, is within just 7 points of Clinton in the Hawkeye State.

Advisers within the Sanders campaign admit that this weekend, but more specifically Sanders' speech at the dinner, is a big moment for him as a candidate.

"We thought the debate was a turning point for him and the Internet and the campaign donations," said Sanders Iowa press secretary Lilia Chacon. "So we hope the same thing happens again, that people exposed to his message will think of him as a viable candidate. But by the same token, we're running our own campaign and we've had so much enthusiasm and such a great vibe going, so much great momentum, that we're doing what we need to do and that's working great for us."

An early test

The event is traditionally held "in the round," meaning that the candidates on stage will be surrounded by the audience. The candidates will give speeches in reverse alphabetical order: first Sanders, then Martin O'Malley, followed by Hillary Clinton. More than 6,000 people are expected to attend, with tickets ranging from $50 to $1,000.

"The Jefferson-Jackson Dinner in Iowa is significant because Iowa's most active Democrats are in attendance. These are the people who make the phone calls, knock on the doors, and generate the enthusiasm for a candidate in their local community. Appealing to these activists with a strong Democratic vision for the future can help galvanize a campaign," longtime Democratic strategist Joe Shannahan told CNN.

There are individual rallies for each candidate leading up to the dinner: President Bill Clinton and Katy Perry will hold a kick off rally for Hillary Clinton supporters Saturday afternoon. O'Malley will serenade his supporters at a meet-and-greet outside the venue. And Sanders is holding a march with supporters across a nearby bridge, in addition to seven dinner watch parties across the state, and held a Friday evening #RockinTheBern concert in eastern Iowa.

The dinner and events surrounding it, Shannahan said, are "an early test of organizational strength for each campaign."

The 2007 dinner was seen by many in the political establishment as a turning point in then-Senator Barack Obama's campaign, catapulting him to an Iowa caucus victory a few months later.

Following last week's debate, Vice President Joe Biden's decision to not enter the race, and a strong performance at the Benghazi hearing, Clinton has had a good week, with strong national poll numbers to boot. She's been widely praised for performances at other Iowa cattle calls, including the Wing Ding dinner in August and the Iowa Democratic Party Hall of Fame dinner in July.

One thing to watch for: Sanders, who almost always speaks from behind a podium, could be challenged by the "in the round" style of the event. Aides have been trying to prepare Sanders for the "performance" element on the moment, especially for speaking in front of the crowd, but always caution that they always want to leave room to let Bernie be Bernie, and not overly coached.

"There's some debate at how he's going to react to it. The thing with him is he doesn't use a teleprompter. He knows this stuff. But he does kind of lean in his way and that is what has made him so easy to parody, but I don't know. He'll do his thing. He knows what his issues are, he knows how to communicate, and we'll see," Chacon said.

'The Revolution Starts Now'

Democratic activist and Iowa Starting Line blogger Pat Rynard, who worked for Clinton in 2008 but has not yet endorsed a candidate, said Sanders now needs to match the enthusiasm and excitement that his campaign has generated with the support of longtime activists and insiders who will be in attendance and can help him win in February.

"The question is, does he vary his message at all? He's been doing very well with the message he's got. It's not conventional, but he's doing better than expected. He needs something a little different on Saturday to go that final distance in Iowa," Rynard said.

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The theme of revolution, a major one throughout the Sanders' campaign, will be one that the campaign doubles down on this weekend in Iowa, with events surrounding the dinner.

When Sanders takes the stage at the dinner, he will be walking out to Steve Earle's "The Revolution Starts Now," according to campaign aides. New campaign signage will be revealed for this weekend's festivities, "The Revolution Starts Now" and "The Revolution Starts Here" (written over a picture of the state of Iowa).

Saturday's events will be key for cementing the support of those key activists.

"After this weekend, it's going to be decision time for Iowa Democrats. Biden is out, there's no one to hold out for. The best activists will be here at the dinner to see the three candidates give one last big pitch, and then you will start seeing a lot of movement in Iowa as people decide who to back," Rynard said.

Andy McGuire, the Iowa Democratic Party chair, said a successful Jefferson-Jackson Dinner performance will come down to who connects with the voters.

"I won't predict what's going to happen, but many times, that night is a good time for really enthusiastic Democrats who really are the workers of our party, and they will look at these candidates and they will many times decide who they think should be the best person to lead this country," she said.