The Trump campaign said Ms Gabbard and Mr Kennedy would serve as honorary transition team co-chairs, along with Trump’s sons and running mate JD Vance.
Presidential transition teams kick into action when a candidate is elected. They help select political appointees and set priorities for the incoming administration.
Trump senior advisor Brian Hughes said that the endorsements of the two former Democrats show how the campaign was earning support “across partisan lines”.
“We look forward to having their powerful voices on the team as we work to restore America’s greatness,” he said in a statement.
On Monday, Trump welcomed Ms Gabbard, 43, to a stage in Detroit, in the key swing state of Michigan.
Ms Gabbard, who represented Hawaii in Congress from 2013 to 2021, quit the Democratic Party in 2022, complaining of its “wokeness”.
She appeared alongside Trump to commemorate US service personnel killed in an attack in Afghanistan three years prior, using it as an opportunity to criticise the Biden administration.
Thirteen US service members and more than 100 Afghans were killed during the suicide bombing at Hamid Karzai International Airport in Kabul.
The incident occurred as American troops made a chaotic withdrawal from the country, and the Taliban returned to power.
Ms Gabbard made frequent criticisms of US military interventionism during her time in Congress.
Appearing at Monday’s event in Michigan, Ms Gabbard said she was appealing to Democrats, Republicans and independents alike to pick Trump at the ballot box in November – saying it was a matter of “saving our country and serving the people”.
Her official backing of Trump marks the culmination of a remarkable political journey over the last decade that began on the progressive left of the Democratic Party.
She was the first Hindu member of the US Congress, going on to serve as vice-chairwoman of the Democratic National Committee – before resigning to endorse the 2016 presidential campaign of Senator Bernie Sanders.
When Ms Gabbard ran for president in 2020, she championed liberal issues like government-run healthcare, free college tuition and gun control.
But Joe Biden became the Democratic nominee that year, and went on to win the presidency. Ms Gabbard endorsed him, but quit the party two years later.
She has since staked out conservative views on issues including abortion and transgender rights, and has become a regular and outspoken guest on Fox News. She has also been accused of spreading “promoting Russian propaganda” by officials in Ukraine.
By early 2024, she was singing the praises of Mr Biden’s rival, Trump, who will this time compete for the White House against Ms Harris.
In the subsequent months, it was even speculated that Ms Gabbard could be in the frame as Trump’s potential running mate for November – a role that ultimately went to Mr Vance.
