From joking that he wished Donald Trump had injected bleach to mocking him for falling asleep in his trial, Joe Biden is doing his best to get underneath his rival's notoriously thin skin.

Biden and his campaign have ramped up personal attacks on the Republican with just six months until November's US presidential election, with the 81-year-old taking an aggressively sarcastic and derisive tone.

In doing so the Democrat is trying to flip the script on a man who also loves to dish out political insults. But Biden's 2024 campaign insists it's not gratuitous and is designed to get across a political message.

"President Biden and our campaign are going to make clear the fundamental choice voters face in this election," Biden campaign spokesperson James Singer told AFP.

"Donald Trump promised to make America great, instead he made America like him: jobless, weak, and diminished," he said. "Joe Biden's campaign is about the American people and the future he believes we can all build together."

Former president Trump has long shown a genius for dominating the news cycle with outlandish statements and political attacks. For years he's mocked Biden over his age, despite being only four years younger at 77, and his lifelong stutter.

Biden is now gunning for his own headlines.

Where Biden's 2020 campaign tried to stay above the fray of petty attacks, this time around he has increasingly gone for open ridicule of Trump, with a marked increase since a fiery State of the Union speech in March.

In April, Biden made the first of a series of jokes about how Trump had suggested injecting bleach to tackle Covid while president during the pandemic. "He missed, it all went to his hair!" he quipped.

The jab couldn't fail to hit home, as Trump is famously proud of his yellow-orange, wraparound hairstyle.

But the jokes have now taken on a darker tone, with Biden joking three times during a fundraising swing in San Francisco and Seattle last week that it was "too bad he didn't" actually inject bleach.

- 'Fight for attention' -

Biden has also branded the scandal-plagued real estate tycoon "Broke Don" in reference to the huge amounts Trump has been spending on legal fees for his criminal indictments instead of on his campaign.

Barely a public event now passes without the Democrat telling a joke about a "defeated-looking guy" approaching him to say he's being crushed by debt, and Biden replying: "Donald, I can't help you."

Then there's "Sleepy Don" -- Biden's revenge for Trump's long-time epithet "Sleepy Joe" -- referring to Trump apparently falling asleep during his hush money trial in New York.

And Biden this weekend branded Trump "clearly unhinged" for being unable to accept his 2020 election loss.

The Biden campaign is meanwhile taking the doctrine of rapid response to new extremes. Within minutes of Trump comments it pumps out often sarcastic emails.

When Trump first went on trial it mentioned the Republican's "Stormy Week" -- a none-too-subtle reference to allegations that Trump ordered secret hush money payments to porn star Stormy Daniels.

Behind the jokes is serious business, Biden's allies say.

Competing against Trump as his trial garners wall-to-wall media coverage meant the Biden campaign had to "fight for attention", a campaign source said.

The "spicier jokes or puns" were not just for the sake of it but rather to highlight the choice between the two candidates.

Amusing comments or stories will get the headlines, but that then eventually gets across Biden's serious messages about issues such as abortion, democracy or the economy, the source said.

And humorous remarks are also more likely to cut through in a fragmented media environment as people get their news through a variety of social media, instead of television as before, they added.

The Trump campaign has dismissed the rival camp's efforts, particularly a Mother's Day video that blasted Trump for his abortion and migrant family separation policies.

Trump campaign spokesman Steven Cheung called the ad "disgusting" and said Biden had a "sad, miserable, cowardly existence."