US President Joe Biden and his predecessor Donald Trump have both passed the delegate thresholds to clinch their parties’ nominations for the election in November.
Four states, one American territory and Democrats living abroad held their primaries on Tuesday.
The result means US voters face a rematch of the 2020 presidential election in eight months.
The nominations will be made official at party 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