DETROIT—President Barack Obama headlined his first and only campaign event with a 2014 Senate candidate this election cycle, just three days before voters head to the polls to decide which party will control the Senate during his last two years in office.

By appearing Saturday night with Democrat Gary Peters, who holds a double-digit lead over Republican Terri Land in public-opinion surveys, Mr. Obama chose to wade into a Senate race in which Democrats already are favored.

“It boils down to a simple question: Who’s on your side? Who’s going to fight for you?” Mr. Obama said to a gymnasium packed with 6,000 supporters at Wayne State University.

“I want you to feel a sense of urgency these last three days,” he said. “When we vote, we win.”

Hours before Mr. Obama’s appearance, Senate Minority Leader Mitch McConnell (R., Ky.) gave a radio address outlining how he would lead Republicans if the GOP took control of the chamber in Tuesday’s electionsand if Mr. McConnell won the race to keep his own seat.