Republicans are lining up to oppose Trump. Will it make a difference?

Donald Trump has a knack for rallying a remarkable range of political opinion around a common goal: preventing his return to the White House.

That now includes prominent names from his own Republican party and top aides who worked under him as president. From former White House officials and national security staff to a once-worshipful press secretary, a host of one-time Trump fans are now lining up to join Democrats in declaring him unfit for another term in office.

White House lawyers who served Republican presidents going back to Ronald Reagan and retired senior military officers have also denounced Trump as a danger to democracy.

Adding to Trump’s humiliation, even members of his own cabinet – who once pledged their fealty with a subservience that would not displease Vladimir Putin – are declining to endorse him for re-election in November.

It’s not entirely a one-way street. For his part, Trump has managed to win over Robert F Kennedy Jr, scion of the US’s most renowned Democratic family and erstwhile presidential candidate, and Tulsi Gabbard, a former Democratic member of Congress.

Related: 2024 US presidential polls: Harris taps into near-record levels of enthusiasm

Trump has added the pair to his transition team but the impact of their endorsement was tempered by Democrats swiftly reminding voters that Kennedy once described Trump as “a terrible president” and his policies as “absurd and terrifying”. Neither will more reasoned Republicans be reassured by Kennedy’s anti-vaxxer views, which have drawn the scorn of at least one major party donor.

The anti-Trump messages are principally aimed at persuading Republicans who may formerly have voted for him that they should put country before party to keep a dangerous populist from a second term in the White House. Opinion polls say that 9% of likely voters who support Trump are prepared to at least consider switching to the Democratic candidate, Kamala Harris.

In early August, her campaign launched “Republicans for Harris” to target voters thought most likely to switch, particularly those who backed Trump’s rival in the primaries, Nikki Haley.

On Monday, 200 Republicans who worked for President George W Bush and the former presidential candidates Senators Mitt Romney and John McCain, released an open letter in support of Harris and her running mate, the Minnesota governor, Tim Walz.

The letter warned that there was more to fear from Trump than a repeat of his first term because he is now bound up with the authoritarian plan to impose rightwing control across the entire US government, including non-partisan federal agencies, known as Project 2025.

“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,” the letter said.

“At home, another four years of Donald Trump’s chaotic leadership, this time focused on advancing the dangerous goals of Project 2025, will hurt real, everyday people and weaken our sacred institutions. Abroad, democratic movements will be irreparably jeopardized as Trump and his acolyte JD Vance kowtow to dictators like Vladimir Putin while turning their backs on our allies. We can’t let that happen.”

The signatories included Jean Becker, President George HW Bush’s chief of staff; David Nierenberg, the campaign finance chair for Romney’s 2012 presidential campaign; and two former chiefs of staff for McCain.

The Trump campaign communications director, Steven Cheung, scorned the letter as written by nonentities.

“It’s hilarious because nobody knows who these people are. They would rather see the country burn down than to see President Trump successfully return to the White House to Make America Great Again,” he told Fox News.

But many Trump supporters will have heard of Stephanie Grisham who served as White House press secretary and routinely appeared on conservative media outlets promoting the then president. Grisham spoke at the Democratic national convention in August where she described how she was once “a true believer” but by the end of Trump’s presidency had concluded that “he has no empathy, no morals, and no fidelity to the truth.

“Trump mocks his supporters. He calls them basement dwellers,” she told the DNC and millions of Americans voters watching on television.

Grisham, who was the first senior White House official to resign after Trump stoked the January 6 insurrection, called on Americans to vote for Harris.

“I love my country more than my party. Kamala Harris tells the truth. She respects the American people and she has my vote,” she said.

A former member of Congress, Adam Kinzinger, who was one of just 10 Republicans to vote to impeach Trump after the attack on the Capitol, also denounced him from the Democratic convention podium for having “suffocated the soul of the Republican party”.

Other Republicans have joined the criticisms of Trump by saying that it is not enough to stand by and not vote in November.

A dozen former White House lawyers who served Republican presidents from Ronald Reagan to George W Bush backed Harris in a letter warning that returning Trump to office “would threaten American democracy and undermine the rule of law in our country”. The letter appeals to “all patriotic Republicans, former Republicans, conservative and center-right citizens, and independent voters to place love of country above party and ideology” and vote for Harris.

“We cannot go along with other former Republican officials who have condemned Trump with these devastating judgments but are still not willing to vote for Harris. We believe this election presents a binary choice, and Trump is utterly disqualified,” it said.

That appears aimed at former officials such as John Bolton, Trump’s former national security adviser, and former defense secretary Mark Esper who have both denounced Trump as unfit for office but declined to endorse Harris.

The letter signed by the 200 former officials said the last presidential election was decided by “moderate Republicans and conservative independents” in swing states who “put country far before party” to defeat Trump four years ago.

“We’re heartfully calling on these friends, colleagues, neighbors, and family members to take a brave stand once more, to vote for leaders that will strive for consensus, not chaos; that will work to unite, not divide; that will make our country and our children proud. Those leaders are Vice President Kamala Harris and Gov Tim Walz,” it said.

Larry Sabato, director of the University of Virginia’s center for politics, said the appeals by the anti-Trump Republicans are aimed at a narrow slice of the electorate given that he doubts many voters are truly undecided.

“A lot of those undecided are just people who are not going to tell the pollster who they’re voting for. There just aren’t that many undecideds. It’s people’s way of avoiding an argument. Also, I don’t think it changes one Republican’s mind who wasn’t open to the idea of not voting for Trump and there aren’t that many of them,” he said.

“But it can make a difference for people who pay attention and who are movable. It also makes a difference for Democratic-leaning independents. It justifies the decision to support Harris they’ve already made. For that reason it matters because these are people, many of them, who’ve worked closely with Trump in the White House and they’ve seen up close how incompetent he is and how unsuited he is for the presidency. It’s critical to get these people on the record.”