"I will be going back to Butler, Pennsylvania, for a big and beautiful rally," former president Trump wrote on his Truth Social site, without providing details on when or where the rally would take place.
Harris, the first Black woman and first Asian American to serve as vice president, swiftly consolidated Democratic support after President Joe Biden tapped her to succeed him on Sunday.
It means so much to have your endorsements, — Kamala Harris (@KamalaHarris) @MichelleObama and @BarackObama.Let's get to work. pic.twitter.com/rAuTyIlCaiJuly 26, 2024
A handful of public opinion polls this week have shown her beginning to narrow Trump's lead.
A Friday Wall Street Journal poll showed Trump holding 49 per cent support to Harris's 47 per cent support, with a margin of error of three percentage points.
A poll by the newspaper earlier in July had shown Trump leading Biden 48 per cent to 42 per cent.
Obama and his wife Michelle endorsed Harris on Friday, adding their names to a parade of prominent Democrats who coalesced behind Harris' White House bid after Biden, 81, ended his re-election campaign under pressure from the party.
"We called to say Michelle and I couldn't be prouder to endorse you and to do everything we can to get you through this election and into the Oval Office," Obama told Harris in a phone call posted in an online video by the campaign.
Smiling as she spoke into a mobile phone, Harris expressed her gratitude for the endorsement and their long friendship.
"Thank you both. It means so much and we're gonna have some fun with this too," said Harris, who would become the nation's first female president if she prevailed in the November 5 election.
Barack Obama, the first Black US president, and Michelle remain among the most popular figures in the Democratic Party, almost eight years after he left office.
Their endorsement could help boost support and fundraising for the Harris campaign and signals Obama is likely to get on the campaign trail for her.
Former US president Donald Trump met with Israeli Prime Minister Benjamin Netanyahu at Mar-a-Lago. (AP PHOTO)
Trump met with Israeli Prime Minister Benjamin Netanyahu at the former president's Florida resort on Friday, a day after Netanyahu sat down with Biden and Harris in separate meetings in Washington.
Trump greeted Netanyahu warmly and told reporters they have a "very good relationship", dismissing any suggestion of tensions between them.
Harris struck a more forceful tone than Biden in her public remarks on Thursday, pushing Netanyahu to help reach a ceasefire in Gaza.
In a televised statement after their talks, Harris said "it is time for this war to end" and expressed concern about the conflict's toll on Palestinian civilians.
Trump called her remarks "disrespectful" on Friday.
The conflict began on October 7 when Hamas militants attacked southern Israel from Gaza, killing 1200 and taking more than 250 hostages, according to Israel.
Israel's retaliatory assault on Gaza has killed more than 39,000, according to Gazan health authorities, and levelled much of the enclave.
The response has displaced most of Gaza's 2.3 million people and created a humanitarian crisis.
The Biden administration has drawn criticism from some Democrats for not pressing Netanyahu more to protect Palestinian civilians.
Trump has also called for a swift end to the war, telling Fox News on Thursday that Israel was getting "decimated with this publicity".
