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 Republican National Committee, for its part, responded to Obama's speech by saying, "President Obama stepped back into the spotlight to make the case that our country is on the wrong track".
Obama is set to appear later Saturday at the Anaheim Convention Center in the heart of Orange County, a once-solid Republican stronghold that voted for Hillary Clinton in the 2016 presidential election that Donald Trump won.
Addressing at the University of Illinois, Urbana-Champaign, less than two months before the mid-term elections that could determine the course of Trump's presidency, he said that the democracy in the U.S. was facing the biggest threat of "indifference" and "cynicism". He's says it's the result of a rapidly changing world and growing inequality and that politicians who appeal to that fear are using "an old playbook".
"It should not be a partisan issue to say that we do not pressure the Attorney General or the Federal Bureau of Investigation to use the criminal justice system as a cudgel to punish our political opponents", Mr Obama said.
Former President Barack Obama is stepping into the midterm battle.
Obama, who is going on the campaign trail Saturday in California, touted the crop of Democratic candidates running for office in November and highlighted women who have stepped up to run on the Democratic side. I don't mean to pretend I'm channeling Lincoln now, but that's not what he had in mind, I don't think when he formed the Republican Party.
For months, the 44th president has been a model of patience and restraint under a barrage of criticism and attacks -- sometimes personal and unfounded -- launched by the man who succeeded him. "And, in a healthy democracy, it doesn't work", Obama said.
California Republicans said Obama's appearance would have little impact and may even help their party.
"We are Americans, we're supposed to stand up to bullies".
At one point, Obama said voters shouldn't stay home just because they "don't feel sufficiently inspired" by a particular candidate. But it is widely accepted - and has been reinforced by primary turnout across the country - that Democrats have been revved up since the day after Trump's inauguration. "I found he's very good for sleeping".
After the speech, Obama will travel to California and campaign for more than a half-dozen House Democratic candidates at an event in Orange County. He will appear at a campaign event in Southern California before heading to OH next week and to IL and Pennsylvania later in the month.
He also criticised Mr Trump's response to the violence previous year at a white nationalist rally in Charlottesville, Virginia, that resulted in the death of one person.
Noting Trump's efforts to improve relations with Russian President Vladimir Putin while USA intelligence has accused Russia of interfering in the 2016 presidential election, Obama asked, "What happened to the Republican Party?" More than six in 10 respondents to a Gallup Poll released in February said they approved of the way Obama handled the job.