US President Joe Biden and his predecessor Donald Trump have both passed the delegate thresholds to clinch their parties’ nominations.
They each won several states in primary elections on Tuesday to propel them over the finish line.
The two 2020 contenders will provide the US with its first rematch in a presidential election for 70 years.
Polling suggests it will be a tight race that will come down to narrow margins in a few key states.
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.
“I believe that the American people will choose to keep us moving into the future,” Mr Biden said in a statement from his campaign.
Incumbency gave Mr 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.
