Joe Biden's all-out attack on Donald Trump in his first 2024 campaign speech said it all — the US president knows he is in for the fight of his political life.

The 81-year-old Democrat has the worst approval ratings of any modern president at this stage in his term and is either neck-and-neck or trailing his likely Republican opponent in the polls.

For months Democrats have been clamoring for Biden's campaign to step up to the plate, as voter concerns mount about the economy and the age of America's oldest president.

Biden did just that on Friday. He launched into a ferocious condemnation of Trump, comparing his rival to the Nazis and branding him "sick," a "loser" and a threat to US democracy.

The question now is whether that strategy — identifying Trump as his main rival even before the Republican nomination process officially starts in Iowa later this month — will be enough.

"If the election were held tomorrow, President Biden would lose," William Galston, senior fellow at the Brookings Institution, told AFP.

Uphill fight

Until now Biden's strategy has focused on positive messages about the president's accomplishments — particularly on leading the economy out of the Covid-19 pandemic. 

But "they haven't moved the needle so far," said Galston, particularly as many Americans are still feeling the effects of high food and housing prices, whatever the improving government statistics say.

Migration through the Mexican border remains a major headache, while there is division in his party over his support for Israel's war on Hamas, and Republicans in Congress are blocking his bid for more funds for Ukraine.

Another major worry is that Biden has been hemorrhaging support among the Black and Hispanic voters who helped him to the White House in 2020.

But perhaps Biden's biggest vulnerability is his age: he has suffered a series of trips and verbal slips that Trump, just four years younger at 77, has repeatedly mocked.

Galston said Biden would have to wage a "vigorous campaign," including multiple trips to battleground states and presidential debates, to convince voters he was not "doddering and senile."

"If he can't wage a vigorous campaign I'm afraid that will simply validate the doubts," he said.

On the economy, Biden would be reliant on wages continuing to rise faster than prices, and a lack of shocks such as a wider Middle East war.

"My overall thesis is that President Biden's campaign is going to have to hope that global events and the domestic economy cooperate," he said.

"Otherwise, that's going to be an uphill fight for the election."

Full Hitler

The Biden campaign now appears to be focusing on his strength as the Democratic Party's only proven "Trump-slayer." 

Biden's speech on Friday, near the symbolic American Revolutionary War site of Valley Forge, was his harshest yet.

It was also the closest he has come to going "full Hitler" — the term younger aides used for directly comparing Trump to the Nazi leader, according to a recent CNN report.

So far Biden has also refused to directly mention the 91 felony counts that Trump faces, in order to avoid the appearance of influencing the judiciary, but that could change if he continues to languish in the polls.

But it remains to be seen whether attacking Trump will turn things around.

"I would say that democracy really is on the ballot. That's Biden's line but I think it's objectively true, it's not hyperbole," said William Howell, a politics professor at the University of Chicago Harris School of Public Policy.

But he told AFP that "most elections are not decided on these kinds of issues, they're decided on the economy, and the incumbent president's approval ratings, whether or not we're at war — these types of material things."

"And while the stakes are very real, it is less clear that talking about them in a full-throated way is the ticket to actually winning reelection for Biden."