Joe Biden cancels campaign event due to illness 'under doctor's orders' Biden was listed as keynote speaker at the Illinois State Fair Democrat Day.

Former Vice President Joe Biden canceled plans to campaign for Democrats in Illinois this week due to an undisclosed illness, state Democrats announced Tuesday.

Biden was scheduled to campaign for Democratic gubernatorial candidate J.B. Pritzker at an event in Springfield, Illinois, on Thursday, but he canceled, citing "doctor’s orders," according to the Illinois Democratic County Chairs' Association.

“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,” Doug House, president of the Illinois Democratic County Chairs’ Association, said late Tuesday. “The cancellation is of course disappointing, but it is clear that the circumstances are simply unavoidable.”

A source with ties to Biden said the illness is not serious, but the former vice president is under the weather and needs a few days’ rest

House did not provide details about the nature of the illness, but he indicated that Biden would “be back campaigning for Democrats in Illinois and across the country in no time.”

Biden was listed as keynote speaker at the 2018 Illinois State Fair Democrat Day.

“While Biden's trip to Springfield is cancelled, our event will continue,” House said. “We remain excited for what will be the largest brunch in our history and look forward to hearing from our incredible slate of Democratic leaders who are ready to energize our party and lead us to victory up and down the ticket this November.”

Biden, a staunch critic of President Donald Trump, has been working hard to improve voter turnout ahead of this year’s midterm election as his party battles to regain control of at least one chamber of Congress.

“It’s up to us, America. Speak out. Rise up. Be heard. The price to be paid for putting our values at risk must be clear. Now,” he said in a late-June tweet. “And show up to vote this November -- in numbers like we’ve never seen.”

Democrats would need to win at least 24 seats to gain a majority in the House, where at least 41 Republicans are planning to retire, resign or run for another office, according to an analysis by ABC News.

ABC News' Arlette Saenz contributed to this report.