“This is not normal. These are extraordinary times and they are dangerous times,” Obama said during the speech at the University of Illinois Urbana-Champaign.

“But here is the good news: in two months we have the chance – not the certainty but the chance – to restore some semblance of sanity to our politics.”

Both parties are urging their core supporters to get to the polls for the November 6 midterm elections, when Democrats need to pick up 23 seats in the House of Representatives and two seats in the Senate to gain majorities in Congress and slam the brakes on Trump’s agenda.

Minutes after his predecessor unleashed his strongest repudiation yet, Trump responded jocularly.

“I’m sorry I watched it, but I fell asleep,” he said.

“I found he’s very good. Very good for sleeping.”

Later, Trump returned to the sentiment during an event in Fargo, North Dakota.

“Isn’t this much more exciting than listening to President Obama speak?” he asked the crowd.

The back and forth between the two titular figures of American politics – each with an unparalleled capacity to both attract his party’s voters and energise the opposition – signalled a dramatic escalation ahead of November elections.

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Obama, kicking off weeks of voter turnout efforts, argued that his aim was not to get into a presidential spitting match, but to convince voters across the ideological spectrum that the conditions that gave rise to Trump’s election were a pressing threat and must be battled directly with increased citizen participation in politics.

“It did not start with Donald Trump,” Obama said.

“He is a symptom, not the cause.”

That did not stop him from denouncing actions that Trump has taken that Obama said undermine American progress, from the ban on travellers from certain Muslim countries to the failure to take action beyond sending “thoughts and prayers” after recent school mass shootings.

He criticised Trump’s attacks on the media, his decision to withdraw from the Paris climate accords, and his government’s response to the 2017 hurricane in Puerto Rico.

He acidly rebuked Trump for his public equivocation about white supremacists involved in a violent confrontation last year in Charlottesville, Virginia.

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“How hard can that be? Saying that Nazis are bad?” Obama asked.

Beyond Trump, Republicans reacted sharply to the speech, arguing that Obama’s decision to return to the political arena could work in their favour.”

“The more President @BarackObama speaks about the ‘good ole years’ of his presidency, the more likely President @realDonaldTrump is to get re-elected,” Senator Lindsey Graham tweeted.

“In fact, the best explanation of President Trump’s victory are the ‘results’ of the Obama Presidency!”

The speech was the first indication of the re-entry the former president and his wife, Michelle, have planned ahead of the midterm elections, a move filled with peril and opportunity as the most powerful duo in Democratic politics test whether they can help handicap Trump’s presidency without also motivating his supporters to go to the polls.

Obama had sought a post-partisan post-presidency, publicly embracing that in a series of events with former Republican president George W. Bush.

And he has been wary of outshining the next generation of liberal leaders, whose success he has described as the greatest ambition of his post-presidency.

“He is acutely aware that he has a mixed record of success when his name is not on the ballot,” said one person familiar with Obama’s thinking.

Having devoted most of his time since leaving office to writing his memoirs and setting up his presidential foundation in Chicago, Obama will be back in the limelight in coming weeks with campaign stops planned in California Saturday, and Ohio on Thursday.

Michelle Obama will also be bringing star power to Democratic races in Las Vegas and Miami late this month.

If Democrats win control of one or both chambers in Congress in November, they would be able not just to stymie Trump’s agenda but to launch investigations of the Trump administration.

Trump told supporters in Montana on Thursday that Republicans needed to maintain control of Congress to stave off possible impeachment proceedings against him, although Democrats have played down any discussion of that approach.

“If it (impeachment) does happen, it’s your fault, because you didn’t go out to vote. OK? You didn’t go out to vote. You didn’t go out to vote. That’s the only way it could happen,” Trump told the rally.

The Washington Post, Agence France-Presse, Reuters