Obama's rebuke of Trump raises US president's hackles

As Barack Obama tore into US President Donald Trump at the Democratic National Convention, his successor hit back with a barrage of angry tweets, saying the former president spied on his campaign and got caught.

Trump went live with his tweeting as Obama and Indian-origin Senator Kamala Harris delivered their speeches at the 2020 Democratic National Convention on Wednesday.

"HE SPIED ON MY CAMPAIGN, AND GOT CAUGHT!" Trump said in an all-cap tweet.

Trump was referring to the charges that his campaign was being spied on in 2016.

As Obama asked Americans to vote for Joe Biden, the presidential nominee of the Democratic Party, Trump fired another tweet in caps.

"WHY DID HE REFUSE TO ENDORSE SLOW JOE UNTIL IT WAS ALL OVER, AND EVEN THEN WAS VERY LATE? WHY DID HE TRY TO GET HIM NOT TO RUN?" he asked.

Obama stayed out of the 2020 primary race and did not endorse until the race was all but over.

Trump continued with his tweets as Harris was delivering her acceptance speech.

"BUT DIDN'T SHE CALL HIM A RACIST??? DIDN'T SHE SAY HE WAS INCOMPETENT???" the president said, referring to the criticism by Harris of Biden during the primaries.

At the start of the third day of the convention, Trump welcomed Obama and former Secretary of State Hillary Clinton to the election campaign.

"Welcome, Barack and Crooked Hillary. See you on the field of battle!" he tweeted.

The president also directed his ire at former first lady Michelle Obama after she took the virtual stage on Monday and accused her of being "over her head" after she encouraged Americans to vote him out of office.

Trump has repeatedly accused Obama of spying on his 2016 campaign. Attorney General William Barr told Congress last year he believed "spying" did take place during a probe into Russian meddling.

Obama launched a scathing attack on his predecessor in his convention speech, saying: "Donald Trump hasn't grown into the job because he can't."

"For close to four years now, 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," he said.

Obama's remarks, live from at the Museum of the American Revolution in Philadelphia, were unusual and even unprecedented in their criticism of a presidential successor.

Obama also used the speech to lay out the case for Biden and his running mate Harris.