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US President Joe Biden and his predecessor Donald Trump have both passed the delegate thresholds to clinch their respective parties' nominations for the election in November this year.
Four states, one American territory and Democrats living abroad held their primaries on Tuesday, BBC reported.
The results all but guarantee a rematch of the 2020 presidential election.
Biden and Trump's nominations will be made official at their party's national conventions this summer.
The 81-year-old president said on Tuesday evening that he was "honoured" voters had backed his re-election bid "in a moment when the threat Trump poses is greater than ever".
Citing positive economic trends, he asserted the US was "in the middle of a comeback" but faced challenges to its future as a democracy, as well as from those seeking to pass abortion restrictions and cut social programmes.
"I believe that the American people will choose to keep us moving into the future," Biden said in a statement from his campaign.
Incumbency gave Biden a natural advantage and he faced no serious challengers for the Democratic nomination.
Despite persistent concerns from voters that his age limits his ability to perform the duties of the presidency, the party apparatus rallied around him.
Meanwhile, Trump, 77, remains very popular with the Republican voter base, which has propelled him to victory in primary after primary over well-funded rivals.
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