The letter from alums of the three Republican presidential nominees prior to former President Donald Trump comes on the heels of a Democratic National Convention last week in Chicago that showcased Republican detractors of the GOP nominee. At least five former aides to former President George H.W. Bush also signed the letter, which has 238 signatures in all.

A similar group of about 150 anti-Trump former staffers of Bush, McCain and Romney pledged support for President Joe Biden in the 2020 election.

"We reunite today, joined by new George H.W. Bush alumni, to reinforce our 2020 statements and, for the first time, jointly declare that we're voting for Vice President Kamala Harris and Gov. Tim Walz this November," the letter reads. "Of course, we have plenty of honest, ideological disagreements with Vice President Harris and Gov. Walz. That's to be expected. The alternative, however, is simply untenable."

Among those who signed the letter in support of Harris and her running-mate, Minnesota Gov. Tim Walz, include: former McCain chiefs of staff Mark Salter and Chris Koch; Joe Donoghue, former legislative director for McCain; Jennifer Lux, press secretary for McCain's 2008 campaign, and Jean Becker, longtime chief of staff for George H.W. Bush.

Also backing Harris are David Nierenberg, Romney's 2012 campaign finance chair; David Garman, under secretary of Energy for George W. Bush; and Olivia Troye, a former advisor to both George W. Bush and Vice President Mike Pence. Troye spoke from the stage of the DNC convention last week.