CHICAGO, United States (AFP) — Barack Obama assailed US Republicans Friday for failing to keep President Donald Trump in check, in a forceful return to the political fray aimed at firing up Democratic voters ahead of key elections.

Since leaving office, the 44th US president has pointedly avoided direct criticism of his successor, making a calculated effort not to utter Trump’s name.

But on Friday the gloves came off.

“What happened to the Republican Party?” asked Obama, accusing Trump of “capitalising” on “fear and anger.”

In a nod to the turbulence of the past week — which saw allegations of a secret “resistance” working inside the White House — the 57-year-old Obama poured scorn on the idea that “everything will turn out okay” because some of Trump’s staff are secretly ignoring the boss’s orders.

“That’s not how our democracy is supposed to work,” Obama thundered, in reference to the revelations by investigative journalist Bob Woodward whose new book describes Trump’s aides battling to rein in an angry, uninformed president.

The Democratic former president assailed Republicans as “unwilling to find the backbone” to challenge Trump head-on — accusing them instead of answering “outrageous” actions with “vague statements of disappointment.”

“They’re not doing us a service by actively promoting 90 per cent of the crazy stuff that’s coming out of this White House, and then saying, ‘Don’t worry, we’re preventing the other 10 per cent,'” he said.

Obama’s remarks to an auditorium of college students in Illinois — the state he represented before he won the White House — marked the opening salvo in a series of campaign stops aimed at boosting fellow Democrats in November’s midterm elections, when voters will elect much of Congress and 36 state governorships.

In a wide-ranging speech punctuated by frequent applause, Obama criticised the divisive politics of the era, decrying Trump’s attacks on the media and the judiciary, his eagerness to work with Russia while neglecting traditional alliances.

But he also sounded a hopeful note about the apparent mobilisation of Democratic voters.

“Out of this political darkness I am seeing a great awakening of citizenship all across the country,” Obama said.

Calling out the president by name, Obama said Trump was a “symptom, not the cause” of broader ailments in the nation’s politics.

The president — who was travelling in North Dakota — took a swipe back at his predecessor after the speech, telling a crowd of supporters at a fundraiser: “I watched it, but I fell asleep.”

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

Taking aim at a key campaign argument by Trump’s Republicans, Obama challenged them for laying claim to a roaring economic recovery that saw 200,000 jobs added last month.

“When you hear how great the economy’s doing right now, let’s just remember when this recovery started,” he said, recalling that he had inherited a dangerous economic downturn and left office amid a recovery.

Trump fired back by pointing at the country’s 4.2 per cent growth rate for the second quarter — accusing Obama of “trying to take credit for this incredible thing that’s happening.”

Had Democrats won control of Congress two year ago, he jeered, “instead of having 4.2 up, I believe, honestly, we’d have 4.2 down.”

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.