CHICAGO • Ending months of self-imposed restraint, former United States president Barack Obama has delivered a blistering critique of President Donald Trump and Republican politics, one that prompted a backhanded dismissal by the man who now occupies the Oval Office.

Over the course of an hour-long address on Friday to kick off weeks of voter turnout efforts for the November mid-term congressional elections, Mr Obama left little doubt about the severity of his concerns over Mr Trump's approach, which he referred to obliquely as "this political darkness".

He compared Mr Trump to foreign demagogues who exploit "a politics of fear and resentment and retrenchment", appeal to racial nationalism and then plunder their countries while promising to fight corruption.

"This is not normal. These are extraordinary times, and they are dangerous times," Mr Obama said during the speech at the University of Illinois at 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."

Minutes after his predecessor unleashed his strongest repudiation yet, Mr Trump responded jocularly, saying he watched it but it made him fall asleep.

Mr Obama 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 Mr 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," Mr Obama said. "He is a symptom, not the cause."

VOTING FOR CHANGE This is not normal. These are extraordinary times, and they are dangerous times. 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. FORMER US PRESIDENT BARACK OBAMA

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That did not stop him from denouncing actions that Mr Trump has taken that Mr 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 Mr Trump's attacks on the media, his decision to withdraw from the Paris climate accords, and his government's response to last year's hurricane in Puerto Rico.

"I know there are Republicans who believe the government should only perform a few minimal functions, but that one of those functions should be making sure nearly 3,000 Americans don't die in a hurricane," Mr Obama said.

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

"How hard can that be? Saying that Nazis are bad?" Mr Obama asked.

Republicans argued that Mr 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.

THE WASHINGTON POST