It is in Andy Levin’s nature to pick fights.

The forthright Detroit congressman and former trade union leader has built a political career on confronting big oil, the gun industry and anti-abortion campaigners.

But as the scion of a distinguished Jewish political dynasty, a committed Zionist and the former president of his synagogue, Levin has been stung by the largest pro-Israel lobby group’s campaign to paint him as an enemy of the Jewish state because he has spoken up for the Palestinians.

The American Israel Public Affairs Committee (Aipac) has spent more than $4m to defeat Levin in next Tuesday’s Democratic primary for a congressional seat in north-western Detroit with a twin strategy to discredit him within the city’s sizable Jewish community while funding an advertising blitz aimed at the wider electorate that avoids mention of the Israel lobby’s involvement.

Aipac and its allies have poured millions of dollars into opposing candidates deemed not to be pro-Israel enough in this year’s Democratic primaries, a move that has alarmed some in the party because much of the money comes from wealthy Trump donors and other rightwing billionaires including Jan Koum, the inventor of WhatsApp, who recently donated $2m.

Levin said Aipac’s involvement also raises the spectre of the entire primary process being hijacked by well-funded lobbies from big oil to the gun industry.

“This strategy to gather millions from rightwing billionaires and other Republican sources to try to determine the outcome of Democratic primaries is deeply troubling. I don’t think the Democratic party can really stand for it and maintain the integrity of our own elections,” he told the Guardian at a campaign rally in the Detroit suburb of Royal Oak.

“It’s Israel today. It could be the fossil fuel industry tomorrow. It could be the tobacco industry, big pharma, any industry or any group that wants to have Democrats who are pliable and will do what they want.”

Aipac boasts that its favored candidates have won in nine of the 10 Democratic primaries it waded into in recent months. Hardline pro-Israel groups have proclaimed these victories as evidence of American voters’ support for its positions. But the campaigns, funded through Aipac’s political action committee, the United Democracy Project (UDP), rarely mention the Jewish state or US policy on Israel.

Other critics note that while Aipac opposes Levin and other candidates deemed to be out of line on Israel by accusing them of working against America’s interests, it has endorsed 37 Republicans who voted against certifying Joe Biden’s victory after the storming of the Capitol on 6 January 2020.

Levin is running for a newly created district after his existing one was scrapped with boundary changes. He is competing against another sitting Democrat, Haley Stevens, whose constituency has also been abolished.

Aipac turned its guns on Levin, a member of the House foreign affairs committee, after he introduced the Two State Solution Act in September, intended to promote a peace agreement including by preventing US aid being used to tighten Israel’s grip on the occupied Palestinian territories, and to block expansion of Jewish settlements and the demolition of Palestinian homes in the West Bank.

The legislation also infuriated some by defining East Jerusalem as occupied territory, which much of the world says it is, when Israel claims sovereignty over the entire city.Aipac portrayed the act as “anti-Israel” and Levin as an extremist.

“Andy Levin represents the fringe wing that is working to undermine the US-Israel relationship,” the lobby group said in an email to supporters last week. “Defeating Andy Levin would remove one hostile voice, but as important, ensuring Haley Stevens wins would cement a pro-Israel champion in the Democratic party.”

Earlier this year, David Victor, a former president of Aipac who lives in the Detroit area, wrote to prospective donors saying that the redistricting “presents a rare opportunity to defeat arguably the most corrosive member of Congress to the US-Israel relationship”.

“To make matters worse, Andy sincerely claims to be a lifelong Zionist, proud Jew and defender of Israel. So when Andy Levin insists he’s pro-Israel, less engaged Democratic colleagues may take him at his word,” wrote Victor.

The email, which raised hundreds of thousands of dollars in support of Stevens, was denounced by other current and former Jewish Democrats in Congress.

“It is fair to disagree on and debate policy approaches. But it is out of bounds to malign the only Jewish candidate in this race by impugning Andy’s love for the State of Israel or his community bona fides, which run strong and run deep,” they wrote in a letter.

The lobby has since poured millions of dollars into supporting Stevens who admits she knew little about Israel until she visited the country three years ago on an Aipac-sponsored trip. Since then she has put an emphasis on “Israel’s right to defend itself” and has expressed scepticism about the US rejoining the Iran nuclear deal.

Levin, on the other hand, comes from a line of politicians with strong ties to Israel including his father, Sander, who served 36 years in Congress, and uncle Carl Levin, who was a US senator for nearly four decades.

Levin accused Aipac of attempting to impose a single hardline view of what it is to be pro-Israel when he has a “proud record” of supporting the Jewish state and a two state solution while also advocating for Palestinian rights. He says his position “has been the position of every Democratic and Republican administration besides Donald Trump”.

“They’re the rightwing Israel lobby. They’re not more pro-Israel than me. In fact, they’re worse for Israel. If you want to be the best friend of Israel, you better give an honest opinion about what needs to happen for your best friend to be secure and safe, and to thrive in the future instead of supporting whatever the Israeli government of the moment does right or wrong,” he said.

“This is the politics of intimidation. When I wrote the Two State Solution Act, I can’t tell you how many of my colleagues came up to me and said, ‘Oh, Andy, I read your bill, it’s great. But of course, I’m not going to co-sponsor.’ They didn’t even need to say why. It’s just assumed that they’re afraid to cross Aipac.”

Much of this fight has gone unnoticed by the average voter in the Detroit district. But they have nonetheless felt its effects.

The UDP has spent heavily on television spots for Stevens that featured President Barack Obama in 2018 praising her work as chief of staff for the taskforce that saw the US auto industry through bankruptcy during the recession. Another focuses on her stand for abortion rights. None of them makes mention of Israel.

Opinion polls show the impact. In February, a Target Insyght poll had the two candidates tied at 41% support each. At the time, Levin led among groups likely to be key to victory, including union workers and women.

A poll by the same company last week showed Stevens at 58% to Levin on 31%. The number of women who said they would vote for Levin fell sharply.

Target Insyght’s director, Edward Sarpolus, said Levin was vulnerable in a number of ways including being a progressive in a new district with a more conservative demographic than his old one.

But Sarpolus said UDP and other pro-Israel lobby money was a gamechanger because it paid for advertising to pound away at those differences in support of Stevens.

“Aipac and the other [pro-Israel] groups kicking in the funds put her over the top because they’re dominating the airwaves with ads and mailings and that type of thing,” he said.

Levin has the backing of a more moderate pro-Israel group, J Street, which has spent about $700,000 to support him with advertising attacking Levin for taking Aipac money.

Aipac is targeting another Detroit Democratic primary race too. The UDP has spent nearly $2m running ads against a member of the Michigan state legislature, Shri Thanedar, who backed a resolution to end the Israeli occupation of Palestinian territories.

Aipac supporters say it is not doing anything other pressure groups do, such as those advocating for abortion rights or environmental policies. But critics say those groups are open about their intent whereas most voters targeted by UDP funded-ads have no idea it is run by Aipac or that it’s goal is to elect a member of Congress it sees as more sympathetic to Israel.

A fellow Democratic member of Congress, Mark Pocan, accused Aipac and its allies of deceiving voters.

“It’s not just dark special interest money that’s spent on candidates. It’s the sketchy, anonymous misleading nature of the money that needs to get called out in the strongest of terms,” he said.

“Now we have groups in Democratic primaries doing this in the most Trojan horse way. First, their values aren’t stated as an organisation. But second they’re raising money from Republican multimillionaires and billionaires to do this. It’s stealth campaigning combined with dirty oppositional money to create a Trojan horse within a Trojan horse.”