More than 100 Republican former national security and foreign policy officials on Wednesday endorsed Kamala Harris for president in a joint letter, calling Donald Trump “unfit to serve” another term in the White House.

Former officials from the presidential administrations of Republicans Ronald Reagan, George H W Bush, George W Bush and Donald Trump, as well as Democrats Bill Clinton and Barack Obama voiced their support for Harris, the Democratic nominee for president in this November’s election. They were joined by some former GOP members of Congress.

The letter said: “We believe that the president of the United States must be a principled, serious, and steady leader.”

It went on: “We expect to disagree with Kamala Harris on many domestic and foreign policy issues, but we believe that she possesses the essential qualities to serve as president and Donald Trump does not. We therefore support her election to be president.”

Among the signees were former defense secretaries William Cohen and Chuck Hagel, who served in the Clinton and Obama administrations, respectively. Others include William Webster, a former CIA and FBI director under the Reagan and first Bush administrations, as well as Michael Hayden, a former CIA and NSA director under the younger Bush and the Obama administrations.

“We firmly oppose the election of Donald Trump. As president, he promoted daily chaos in government, praised our enemies and undermined our allies, politicized the military and disparaged our veterans, prioritized his personal interest above American interests, and betrayed our values, democracy, and this country’s founding document,” the letter added.

Pointing to Trump’s involvement in the 6 January 2021 insurrection at the US Capitol, his “susceptibility to flattery and manipulation” by authoritarian leaders such as Vladimir Putin of Russia and Xi Jinping of China, and “chaotic national security decision-making”, the former officials called Trump unfit to serve again as president or in “any office of public trust”.

The former officials also pointed to Harris’s support for Nato and Israel, as well as her commitment to signing the bipartisan border security package that Republicans blocked, and her pledge to appoint a Republican to her administration as reasons for their endorsement.

Several former Trump officials who signed the letter include Mark Harvey, a former special assistant to the president, and Elizabeth Neumann, a former assistant secretary of Homeland Security.

In recent weeks, a handful of Republicans have crossed party lines to endorse Harris, including the former Virginia representative Barbara Comstock. In an interview with CNN, Comstock explained her decision, saying: “After January 6, after Donald Trump has refused for four years to acknowledge that he lost [the 2020 election], and his threats against democracy, I think it’s important to turn the page.”

Other Republicans who have endorsed Harris include Alberto Gonzales, a Republican attorney general who served under the W Bush administration, the former Illinois representative Adam Kinzinger, as well as Trump’s former press secretary Stephanie Grisham and communications director Anthony Scaramucci.