US President Barack Obama has implored voters to help continue his legacy by carrying Hillary Clinton to victory at the polls in November.
Mr Obama praised Mrs Clinton at the Democratic convention in Philadelphia, as the most qualified person ever to run for the White House.
He said voters faced a choice between hope and fear, attacking "home-grown demagogue" Republican Donald Trump.
Mr Trump responded by rejecting the president's optimistic portrayal.
"Our country does not feel 'great already' to the millions of wonderful people living in poverty, violence and despair," he said on Twitter.
Mr Obama recounted his key victories over his past eight years in office and painted an optimistic future.
He extolled Mrs Clinton's character, calling her a "leader with real plans to break down barriers, blast through glass ceilings and widen the circle of opportunity to every single American".
Mr Obama described his nation as "full of courage", "decent and generous", but also concerned about racial divisions and "frustrated with political gridlock".
The optimistic high ground - BBC's Anthony Zurcher
President Obama is good at this. He's really, really good at this.
For all his flaws - and conservatives will be quick to point them out - he's always been able to deliver a pitch-perfect speech on the biggest stages, and this was no exception.
It was the kind of speech that had some conservatives shaking their heads, wondering how their party ceded the optimistic high ground to their opponents. Mr Obama even quoted Ronald Reagan's "shining city on a hill" line, if only to make the political shift all the more clear.
When he mentioned Mr Trump, there were boos. Mr Obama said: "Don't boo, vote."
He said Mr Trump "calls himself a business guy, which is true, but I have to say I know plenty of businessmen and women who have achieved remarkable success without leaving a trail of lawsuits and unpaid workers and people feeling like they got cheated."
Mr Obama added: "Does anyone really believe that a guy who spent his 70 years on this Earth showing no regard for working people is suddenly going to be your champion, your voice? No."
He said Mr Trump's grim portrait of America was "not the America I know"