Former U.S. President Barack Obama forcefully rebuked the Republican Party on Friday, criticizing it for divisive politics and its retrenchment in global leadership.

"What happened to the Republican Party?" Obama asked in a speech at the University of Illinois.

"They're undermining our alliances, cozying up to Russia," he said.

The party's "central organizing principle in foreign policy was the fight against communism and now they're cozying up to the former head of the KGB."

The former president's remarks -- which marked his full return to the political stage -- came ahead of series of campaign stops to help fellow Democrats running in pivotal upcoming midterm races.

Democrats are hoping to ride an anti-Donald Trump "blue wave" to gain seats and secure control of the House of Representatives, the lower chamber of Congress, and are also battling for seats in the Senate.

The speech in Obama's home state was a preview of the arguments he is expected to make on the campaign trail.

He criticized the divisive politics of the Trump era and took aim at recent Republican policy moves such as the $1.5 trillion in tax cuts that ballooned the nation's budget deficit.

"It's supposed to be the party... of fiscal conservatism. Suddenly deficits do not matter," Obama said.

The former president implored an audience of college students to vote in the upcoming election. Historically, midterms are lightly attended and Obama made a point of reminding his audience that only one in five young people voted in the 2014 midterm election.

"The biggest threat to our democracy is cynicism," Obama said.

"You've got to do more than retweet a hashtag, you've got to vote."