NEW YORK • Former United States presidential candidate Hillary Clinton has endorsed fellow Democrat Joe Biden's campaign for the office she once sought.

"This is a moment that we need a leader, a president like Joe Biden," she said on Tuesday during a joint event online with the former vice-president. "Think of what it would mean if we had a real president, not just somebody who plays one on TV, but somebody who gets up every morning worried about the people that he's responsible for leading during this crisis."

The endorsement, while no surprise, marks the latest in a series of high-profile Democrats lining up behind Mr Biden ahead of what is expected to be a tight Nov 3 election rematch between their party and Republican President Donald Trump.

Mr Biden, who was former president Barack Obama's vice-president, won Ohio's Democratic presidential primary on Tuesday in a mail-in-only contest that replaced regular voting amid the coronavirus pandemic.

The Associated Press declared Mr Biden the winner with 74 per cent of the vote, with just a few precincts reporting.

The primary had been scheduled for March 17, but was postponed due to concerns about safety and eventually conducted only by mail.

The state has 153 delegates, roughly 4 per cent of the pledged delegates to the Democratic National Convention.

The postponement of many primaries - and New York state's outright cancellation - means that the total number of delegates could be different by the time of the convention.

After Mr Bernie Sanders dropped out earlier this month, Mr Biden is left with no significant challenger for his party's nomination at the convention, which was also pushed back and is now scheduled for late August in Milwaukee, Wisconsin.

Senator Sanders had bitterly clashed with Mrs Clinton when they contested their party's 2016 primary.

Mrs Clinton, a one-time US secretary of state, former senator from New York and wife of former president Bill Clinton, lost the 2016 presidential race against Mr Trump in an election that is still the source of anger and consternation among many liberals who expected to elect the first female president but also wrestled with whether they chose the right candidate.

The endorsement from Mrs Clinton came at a Biden campaign town hall focused on the impact of the Covid-19 outbreak on women, who are expected to play a critical role in the most competitive swing states in the election.

Mr Biden introduced Mrs Clinton as the woman who should be president now. Both simultaneously considered presidential runs, but he ultimately decided against mounting a campaign then as he grieved over the death of his son Beau.

Mr Obama endorsed Mr Biden's current campaign two weeks ago.

REUTERS, BLOOMBERG