Third parties could hurt Donald Trump as much as Joe Biden in the 2024 presidential election, the group No Labels has claimed, insisting that its contentious effort to build a unity ticket cannot be compared to spoiler candidates of the past.

“We’re not spoiling anything,” No Labels’ Jay Nixon, a lifelong Democrat who spent 30 years serving Missouri as a governor, attorney general and state senator, told the Guardian.

The centrist group is preparing for the strongest third-party presidential bid since the Texas businessman Ross Perot earned nearly 19% of the popular vote in 1992. It has gained ballot access in 12 states so far. But No Labels’ critics in the Democratic party have warned that it could siphon votes that would otherwise go to Biden.

They point to Green party examples such as Ralph Nader, who in 2000 polled at less than 3% but was widely blamed for depriving Al Gore of critical votes, and Jill Stein, who in 2016 notched just 1% but may have made all the difference in battleground states that Hillary Clinton lost by a similar margin.

Ryan Clancy, chief strategist at No Labels, told the Guardian: “There is no parallel between what those two did and what we’re doing, the reason being Ralph Nader and Jill Stein were leftwing protest candidates. The only votes they were ever going to pull were going to be votes that otherwise went to Gore or Hillary Clinton.”

Clancy pointed to a recent USA Today newspaper opinion poll that showed the leftwing independent candidate Cornel West eating into Biden’s support but rival Robert Kennedy Jr taking votes from Trump. He said: “The takeaway there is whether the independent pulls from one side or the other depends on who the independent is.

“Some are bad for Biden, some are bad for Trump, some it’s a wash. The problem we have is that for a year now the people attacking us just keep hammering home if an independent runs, it by definition can only help Trump and we don’t agree with them.”

Some Democrats argue that, while the principle of a multi-party democracy is admirable, now is not the time because Trump poses a fundamental threat to democracy itself. The Republican frontrunner faces criminal charges over his role in the January 6 insurrection and recently teased that he would be a dictator only on “day one”.

No Labels, founded in 2009 by Nancy Jacobson, a prominent Washington political fundraiser, contends otherwise, pointing to recent polls that show Biden trailing Trump nationally and in swing states.

Nixon, its director of ballot integrity, said: “For somebody who’s behind in a race to say that you’re going to cost them a race you’re already behind in doesn’t make sense to me. I live in a number world and if you’re behind and you say it’s somebody else’s fault?

“The president’s running against somebody who’s been indicted four times in four separate courts, and the president’s numbers have gotten worse. This is why mirrors are useful. You got to look at where you are what you’re going to do. We’re not spoiling anything.

“We’re providing for the vast majority of Americans an opportunity that they really want. If you’re ever out with our signature gatherers, Americans want this choice and we’re working hard in this phase of it to do what we can professionally to give them that choice.”

Asked why he is not campaigning for Biden’s re-election, Nixon replied: “This is not about me saying bad things about Joe Biden. I like the president and respect him tremendously, much more than Donald Trump.

“I don’t want to get into each one of them is equal in that sense because to me Trump is so far out that he’s checked out of my head. Biden, on the other hand, his time for service is coming to an end and I appreciate the service he’s provided.”

Polling has shown that most Americans have no appetite for a Biden v Trump rematch. The current president’s approval rating sits in the 30s while his predecessor’s hovers in the low 40s. Although no third party candidate has ever come close to winning a presidential election, No Labels claims that its polling shows a unity ticket could realistically prevail in 25 states – enough to win the electoral college.

Clancy said: “The reason there’s an opening is because somebody like Trump is on the ballot. If the Republicans and Democrats were putting up broadly appealing candidates, there would be no opening.

“One thing No Labels has said from the beginning is, if there is not an opening, we are not hellbent on doing this and offering up our ballot line to a ticket. It is only because the public is so profoundly unhappy with the choices they’re getting that there’s an opening for something.”

He added: “There’s a lot of people in the Democratic party that have been spending a disproportionate amount of time attacking No Labels for what we’re doing. The time would be much better spent either thinking one, how do we get a better candidate or, in the absence of that, how do we make a more persuasive appeal to the voters? Because they’re not doing well on either score at the moment.”

No Labels says it no longer takes corporate donations and does not release the names of its financial backers because they have a right to privacy. It is still working on the details of how it will choose a nominee and gathering feedback from people who will be delegates at a virtual convention.

Former elected officials in both parties now affiliated with No Labels include Senator Joe Manchin, who has announced that he will not seek re-election, former governors Jon Huntsman, Larry Hogan and Pat McCrory and ex-senator Joe Lieberman, a Democrat who became an independent late in his political career.

But two Democratic groups, the centrist Third Way and more progressive MoveOn, have been mounting an aggressive campaign to keep No Labels off ballots and minimise its impact. Nixon claims to be unfazed. “Like the old helicopter pilots in war used to always say, if you’re getting flak, that means you’re over the target.”

Clancy pointed out that there have been two lines of attack on No Labels. “One of which is: you know what, even if you guys think you’re doing the right thing, the unintended consequence of what you’re doing is you’re going to help Trump. I don’t agree with that, but OK, we can have a debate about that.

“There’s another line of argument which is: look, you want to help Trump, that’s the secret goal, which is so stupid, because for starters, I came up in Democratic politics – I used to work for Joe Biden – and I don’t ever want to see Donald Trump in the White House again.

“For me, there’s no equivalency between the two of them. Governor Nixon said the same thing. All our co-chairs have said that. Why would these people who have multi-decade careers in civil rights and public service put their whole reputation on the line so they can be part of a secret cabal to help Trump? It doesn’t pass the stupid test but that doesn’t stop our opponents from saying it.”

However, Third Way continues to provide vehement opposition to No Labels’ effort to run a unity ticket. Matt Bennett, its co-founder and executive vice-president for public affairs, argues that in key states there are people who would vote for Biden in a two-way race with Trump but who might not if presented with a third option.

“They do run the risk of imperiling American democracy in ways that we have not experienced since the American civil war,” Bennett said. “There has never been a threat like a second Trump term to the fundamental basics of the American republic.

“If this were like [Barack] Obama against [Mitt] Romney, we might not support a third party but we wouldn’t be freaking out about it. The reason we are so panicked is because this is basically the only way that Trump can return to the White House and if he does that it will be a world historic catastrophe. And they are threatening that.”