In the 2024 cycle, those crucial battlegrounds look set to be Arizona, Georgia, Michigan, Nevada, Pennsylvania and Wisconsin.
Biden won all six to secure a 306-232 “electoral college” victory over Trump in 2020. But Trump now leads opinion polling averages in all six.
Pennsylvania
It was once reliably Democratic but these days they don’t come any closer than the Keystone State.
Trump won Pennsylvania by 0.7 per cent in 2016 and Biden claimed the mid-Atlantic battleground by 1.2 per cent in 2020.
Before Trump, it had a quarter-century unbroken run of voting for Democrats.
Known for its “rust belt” cities like Philadelphia and Pittsburgh, Pennsylvania has been blighted for decades by the steady decline of its industrial manufacturing base.
Biden has visited to tout recent major infrastructure wins. Trump is courting the rural white population with promises of an inflation-free future.
Georgia
The southeastern state of Georgia was a flashpoint for elections in the Trump years, and the controversy simmers.
Prosecutors there indicted Trump in an election interference case after he called state officials following the 2020 election and urged them to “find” enough votes to overturn Biden’s narrow victory.
Trump faces multiple felony charges, including a racketeering count at the heart of the case against the ex-president and more than a dozen co-defendants.
Biden, benefiting from a large proportion of African-American voters, was the first Democrat to win the Peach State since 1992.
Michigan
Trump caused an upset by carrying Michigan – a former Democratic stronghold – on his way to defeating Hillary Clinton in 2016.
Biden put the state back in the blue column in 2020, buoyed by unionised workers and a large black community that helped deliver a victory margin of less than three percentage points.
But this time the Democrat risks losing the support of a 200,000-strong Arab-American community that has denounced his handling of the Israel-Gaza war.
Arizona
As Pennsylvania leaned leftward, Arizona went right – making Biden the only Democratic candidate it has opted for this century.
It was one of the tightest races of 2020, with Biden triumphing by just 10,457 votes.
The defeated Trump hopes frustrations over his victor’s immigration policy will swing the Grand Canyon State, which shares a border with Mexico, back in his favour.
Super PAC American Values says it has already secured enough signatures to get Robert F. Kennedy Jnr on the ballot, which could complicate either main party candidate’s prospects.
Biden-Trump sequel: a look at US presidential rematches through history
Biden-Trump sequel: a look at US presidential rematches through history
Nevada
Nevada – the swing state with the smallest proportion of college graduates – hasn’t voted for a Republican since George W. Bush in 2004 but conservatives are convinced they can strike a blow for their cause in November.
So far, polls are proving them right, with the historically Democratic Latino community increasingly leaning Republican in recent election cycles.
A Morning Consult/Bloomberg poll gave Trump a six-point lead in February but it is another state where Kennedy aims to be competitive and Trump’s lead widens slightly in a three-way race.
Wisconsin
Hillary Clinton was rewarded with defeat in Wisconsin after giving the state a wide berth during the 2016 campaign.
As with Midwestern neighbour Michigan, it was a different story when Trump’s opponent was Biden, who turned a 23,000-vote deficit into a 21,000 winning margin for Democrats.
Polls suggest Wisconsin will be one of the closest 2024 races. Trump’s lead in the RealClearPolitics average is just one per cent – well within the margin of error.
Republicans see the state as a winnable prize and key target, with the party holding its summer convention in the largest city, Milwaukee.