President Barack Obama on Wednesday implored Americans to help Hillary Clinton beat Donald Trump, warning the 2016 race was not just about politics, but the nature of US democracy.
Speaking before a rapt Democratic convention in Philadelphia, Obama praised Clinton's caliber and readiness for the White House while painting her Republican rival as a know-nothing authoritarian.
The election, Obama said, was "not just a choice between parties or policies; the usual debates between left and right."
"This is a more fundamental choice," he said.
In the city where America's founding fathers signed the Declaration of Independence, Obama declared the November election a "fundamental choice" about what the country is and the very "meaning of our democracy."
Obama spearheaded a day-long effort by Democrats to depict Trump as unfit to be commander-in-chief, contrasting the political neophyte with his more experienced Democratic rival.
Trump, he said, was "betting that if he scares enough people, he might score just enough votes to win this election."
The wealthy real estate mogul has run on a hard-right platform of banning Muslims from entering the United States and building a border wall to keep Hispanic migrants out.
Earlier in the day, he launched a pre-emptive attack against Obama, calling him "the most ignorant president in our history."
The president fired right back.
Punching at Trump's campaign slogan, he insisted "America is already great. America is already strong. And I promise you, our strength, our greatness, does not depend on Donald Trump."
"Our power doesn't come from some self-declared savior promising that he alone can restore order as long as we do things his way. We don't look to be ruled."
Voicing optimism about the future, Obama said "anyone who threatens our values -- whether fascists or communists or jihadists or homegrown demagogues -- will always fail in the end."
Acting as a character reference for his secretary of state of four years, with whom he battled in the 2008 primaries, Obama said Clinton was uniquely qualified to succeed him.
He said the 68-year-old former first lady and senator knew what it's like to be in the room when tough epoch-making and presidency-breaking decisions were taken -- like the move to strike Osama bin Laden.
"I can say with confidence there has never been a man or a woman -- not me, not Bill (Clinton), nobody -- more qualified than Hillary Clinton to serve as president of the United States of America," he said.
At the end of his speech, Clinton -- the first woman to win the presidential nomination of a major US political party -- joined him on stage to deafening shouts from the party faithful. The pair embraced and waved to the crowd.
Earlier in the day a series of Democrats had sought to zero in on disunity in the Republican party and zero in on disgruntled voting blocs.
Vice President Joe Biden trashed Trump's claim to be a champion of the middle class, laying into the mogul's reality television show catchphrase: "You're fired!"
"Think about that," said Biden, his voice growing angrier. "Think about that. Think about everything you learned as a child no matter where you were raised. How can there be pleasure in saying 'You're fired'?
"He's trying to tell us he cares about the middle class? Give me a break! That's a bunch of malarkey!"
Clinton's vice presidential running mate Tim Kaine made a folksy appeal to more centrist voters, deriding a "slick-talking, empty-promising, self-promoting one-man wrecking crew!"
A host of Democrats zeroed in on Trump's comments that seemed to invite Russian cyber espionage against Clinton.
"As someone who was responsible for protecting our nation from cyber-attacks, it is inconceivable to me that any presidential candidate would be that irresponsible," said former CIA director Leon Panetta.
It was left to Obama to draw all these disparate strands together into a comprehensive argument as to why Clinton should succeed him.
Now in the twilight of his second term, Obama faces ever-dwindling opportunities to address the nation, mold his legacy and influence the 2016 race.
But on Wednesday, he had a prime-time chance, appearing before thousands of delegates and tens of millions of viewers at home.
The White House said Obama had been working on the roughly 45-minute speech for weeks.
Yet this touchstone presidential moment had been a decade or more in the making.
The address bookends Obama's career-launching address to the Democratic convention in 2004, his contentious 2008 primary battle with Clinton and his eight years in office.
Obama made a familiar case for what has been achieved during his two terms, highlighting America's recovery from the Great Recession, the Iran nuclear deal and reforms to health care.
But this was also a passing of the party torch.
"I have confidence as I leave this stage tonight that the Democratic Party is in good hands," Obama said, appearing to become emotional.
"Thank you for this incredible journey," he said before his long exit from the stage.