He dubbed Mr Obama as a "terrible" chief executive and said his "ineffective" two terms were the reason voters put him in the White House four years ago. "I see the horror that he's left us. The stupidity of the transactions that he made," Mr Trump replied to a reporter during a coronavirus briefing.

"How bad he was," the 45th president said of the 44th, calling Mr Obama "so ineffective" and "so terrible." The president then contended if Mr Obama and former Vice President Joe Biden, the designated 2020 Democratic presidential nominee, had not done such a poor job, he would not have been elected in the first place. Mr Trump ran largely on a complete repudiation of the Obama administration's raft of domestic and foreign policies. His comments came in response to what Mr Obama will say later in the night during the Democratic Party's 2020 nominating convention. "I did hope, for the sake of our country, that Donald Trump might show some interest in taking the job seriously, that he might come to feel the weight of the office and discover some reverence for the democracy that had been placed in his care," Mr Trump will tell voters around 10:30 p.m.

"But he never did. He's shown no interest in putting in the work, no interest in finding common ground; no interest in using the awesome power of his office to help anyone but himself and his friends, no interest in treating the presidency as anything but one more reality show that he can use to get the attention he craves," the former president will say. Read more about Mr Obama's speech here. Speaker after speaker during the opening two nights of the convention have hammered Mr Trump, saying he has "quit" on the United States with a disinterested coronavirus response and a presidency geared solely for his conservative political base.

Barack Obama in a stunning prime-time address to the nation he once led -- warned that his successor was on the cusp of destroying democracy itself. The latest instalment of the long duel between Barack Obama and Donald Trump perfectly exemplified the jarring contrasts in personal and political temperaments of the two defining White House residents of this age. And it took their rivalry to a level unprecedented in the modern history of the presidency.

Obama -- serious and intellectual -- delivered a complex constitutional lecture on primetime television during the virtual Democratic National Convention. He summoned historic sweep, encompassing the Founders, the Civil Rights Movement, America's immigrant heritage and young Americans he called to action today to save their freedoms just as their ancestors had done every time the country's promise was imperilled.

Donald Trump meanwhile, back at the White House, was rage-tweeting in real-time in all-caps, flinging wild accusations and lies that, if anything, provided contemporaneous evidence of his predecessor's sombre warnings. In 2004, Obama made his name with the youthful, exuberant -- and perhaps naive -- hope of his address to the DNC. Sixteen years on, a grizzled Obama the elder warned America that it should expect a "president to be the custodian of this democracy."

"We should expect that regardless of ego, ambition, or political beliefs, the president will preserve, protect, and defend the freedoms and ideals that so many Americans marched for and went to jail for; fought for and died for," he said. Obama, now a private citizen, has no formal power and no mandate to speak to the American people the way he did Wednesday night, at a virtual convention that in its crowd-free isolation served to underscore the grave nature of his message.

But he carries the authority of his historic status, the moral weight of two White House terms and enduring popularity in the half of the country revolted by Trump's abuses of power, divisive racial politics and constant cultivation of his own ego. And Obama's breach of etiquette for retired presidents was preceded by years of Trump attacking him in ways never previously seen by an American presidential successor, with Trump routinely, baselessly accusing Obama of treason. The former President presented himself as a guardian of democracy, of 243 years of Constitutional norms and authority borrowed from the masses not imposed from a strongman leader from above. "Do not let them take away your power.

Don't let them take away your democracy," Obama said in a plea that was far deeper than a political leader's repudiation of the legacy destroying policies of his predecessor. "Make a plan right now for how you're going to get involved and vote. Do it as early as you can and tell your family and friends how they can vote too," Obama said, accusing the Trump administration of suppressing the vote and counting on the cynicism of the people to guarantee four more years. "What we do echoes through the generations," Obama said, in what amounted to probably the most-watched lecture on America's constitutional heritage in history.