"Delete your account." Hillary Clinton's snarky tweet Thursday to her White House rival Donald Trump was among the funniest of the campaign. Almost as funny as it was awkward.

The message by the 68-year-old secretary of state -- actually penned by a young staffer, a Clinton aide told AFP -- was in response to the latest broadside by Trump, who had commented on how "Obama just endorsed Crooked Hillary" on Thursday.

Social media ate it up, and within 90 minutes the message was retweeted 145,000 times, making it "now the most retweeted tweet of the campaign!" according to Clinton's social media director Alex Wall.

With "Delete your account," Clinton, who has struggled to connect with young voters, embraced the quick-witted dry humor of America's millennials.

But it also triggered ferocious comebacks from her critics over use of a private email account while she served as America's top diplomat -- a scandal she has been unable to shake.

Republicans including Trump have savaged her for her judgment, saying she put US national security at risk, and have accused her of deleting key emails that she did not want Americans to know about.

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