US President Joe Biden and his predecessor Donald Trump have both passed the delegate threshold to win their party's nominations for the November election.
Four U.S. territories and overseas Democrats held their primaries on Tuesday.
The result means American voters will face a rematch of the 2020 presidential election in eight months.
Candidates will be officially decided at a party convention this summer.
The 81-year-old president said Tuesday night that he was “honored” by voters' support for his re-election “at a time when the threat from President Trump is greater than ever.”
Citing positive economic trends, he said the United States is “on the road to recovery,” but faces challenges to its future as a democracy, as well as those calling for passing abortion regulations and cutting social programs. claimed to have done so.
“We believe the American people will choose to continue moving us forward into the future,” Biden said in a statement from his campaign.
Incumbent status gave Biden a natural advantage and he faced no serious challenger in the race for the Democratic nomination.
The party machine rallied behind him despite persistent concerns from voters that his age limited his ability to carry out presidential duties.
Meanwhile, the 77-year-old Trump remains highly popular among Republican voters, winning primary after primary over his well-funded rivals.
His campaign for a second term in the White House has focused on toughening immigration laws, including promises to “close the border” and implement “record-breaking” deportations.
Trump also vowed to fight crime, increase domestic energy production, tax foreign imports, end the war in Ukraine and resume an “America first” approach to world affairs.
Both riders have dominated the race so far, so Tuesday night's result is no surprise.
US presidential primaries and caucuses are state-by-state competitions to win the most party delegates.
Learn more about US elections
Democratic and Republican primaries have slightly different rules, but the process is essentially the same.
Each state is allocated a certain percentage of party delegates, which can be awarded to the winning candidate overall or proportionally based on the results.
Republican candidates need to secure at least 1,215 delegates during the primary period to win the presidential nomination, while Democrats need to secure 1,968.
On Tuesday, Republicans held primaries in Mississippi, Georgia and Washington states, and held caucuses in Hawaii.
Meanwhile, the Democratic Party held primaries for Democrats in Georgia, Washington and Mississippi, as well as the Northern Mariana Islands and overseas.
Mr. Biden and Mr. Trump's main competitors had dropped out before Tuesday's primary, and the outcome was all but certain.
Trump's last remaining rival, former United Nations Ambassador Nikki Haley, resigned earlier this month after losing 14 states to Trump on Super Tuesday.
Several more states have yet to hold primaries, but the 2024 general election is effectively underway, with Trump and Biden exceeding the delegate threshold.
The US presidential election will take place on November 5, 2024.