SPRINGFIELD — Former Vice President Joe Biden is “just under the weather.”

That was the official reason Biden’s office gave Wednesday after the former vice-president cancelled his trip to Illinois to speak at the State Fair, where Democrats hope he would help unleash a “Blue Wave” across the state.

Biden had been scheduled to serve as a keynote speaker on Democrat Day at the fair on Thursday to help bolster support for Democratic gubernatorial candidate J.B. Pritzker and other Democrats in the state.

In announcing Biden as their keynote speaker earlier this month, the Illinois Democratic County Chairs’ Association said Biden and other speakers would “rally, motivate and inspire Illinois Democrats to execute the battle plan to ensure a ‘Blue Wave’ in every county of our state.”

But Doug House, the association’s president, said in a statement Tuesday night that Biden is “under doctor’s orders not to travel.”

On Wednesday, Biden’s office said the illness is “nothing serious.” A spokesman said Biden is “just under the weather and needs a few days’ rest.” His office said the former vice president had hoped to be at the event and will be “back in the arena shortly.”

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Biden was scheduled to keynote the County Chairs’ Association brunch at a Springfield hotel. The brunch kicks off the day’s activities. The association on Wednesday morning announced South Bend, Indiana, Mayor Pete Buttigieg will replace Biden as the headliner.

Buttigieg, 36, is a Harvard graduate and veteran of the Afghanistan war. Buttigieg is openly gay, and his name has been thrown into the mix of potential presidential contenders for the 2020 election.

The State Journal-Register in Springfield reports the 2,500 tickets for hotel’s main ballroom had been sold to the Biden event. Demand for tickets had prompted use of a second room that holds 700 people.

“Everyone who knows Vice President Biden knows that he gives our party and our country his all, but unfortunately he is sick and is under doctor’s orders not to travel,” the House statement said.

“The cancellation is of course disappointing, but it is clear that the circumstances are simply unavoidable. We all hope he gets well soon and I’m sure he’ll be back campaigning for Democrats in Illinois and across the country in no time,” House said.

House said Democrat Day activities will still go on, although Biden — named as a possible Democratic presidential contender — will be missed at Thursday’s events, which were intended to provide a boost to Pritzker’s campaign.