More than 240 Jewish American voters in Pittsburgh have signed a letter denouncing the US’s largest pro-Israel group for backing extremist Republican election candidates while spending millions of dollars to oppose a Democrat who would be Pennsylvania’s first Black female member of Congress.

The letter condemned the powerful American Israel Public Affairs Committee (Aipac) for its attempts to defeat Summer Lee, a candidate for the district that includes Pittsburgh, after failing to block her during the Democratic primaries earlier this year because of her criticisms of Israel’s oppression of the Palestinians.

The signatories said they were “outraged that at this critical moment in American history, Aipac has chosen to cast Democrats like Lee as extremists” while endorsing more than 100 Republican candidates who voted to overturn the 2020 presidential election.

The letter suggested that Aipac does not represent the views of the majority of American Jews and is working against their interests by also endorsing Republicans who promote white supremacy, a particularly sensitive issue in a city where 11 worshippers at the Tree of Life synagogues were murdered in an antisemitic attack four years ago.

“We also condemn Aipac endorsement of lawmakers who have promoted the antisemitic ‘Great Replacement’ conspiracy theory that helped inspire the murder of eleven members of the three synagogues housed at Tree of Life,” the letter said.

“Clearly, their definition of ‘extreme’ is completely opposite to that held by the majority of American Jews – who worry about the stark rise in antisemitism and white nationalism in our state and in our country.”

It is the first time Aipac has funded support for a Republican contender for Congress over a Democrat in a general election, marking a further shift away from its once more bipartisan approach.

Aipac’s campaign funding arm, the United Democracy Project (UDP), is paying hundreds of thousands of dollars for television advertising and mailings against Lee. The group is backing Mike Doyle, who supports a federal ban on abortion and has described himself as very conservative.

The UDP has posted a leaflet to voters calling Lee “too extreme” because of her positions on police, prison and immigration reform. The leaflet makes no mention of her criticisms of Israeli government policies which do not appear to be an election issue for most voters, although Aipac has previously said that its “sole factor for supporting Democratic and Republican candidates is their support for strengthening the US-Israel relationship”.

Lee has drawn Aipac’s fire for her support of setting conditions for the US’s considerable aid to Israel, for accusing Israel of “atrocities” in Gaza, and for drawing parallels between Israeli actions against Palestinians and the shooting of young black men in the US.

In a tweet earlier this week, Lee accused Aipac of funding extremists: “8 days from making history in PA–where Black women have never had federal representation–Aipac is funding my extreme GOP opponent. Since endorsing 100+ insurrectionists, Aipac has repeatedly shown us that democracy has never been as important as keeping progressives out.”

Lee’s campaign has an additional cause for concern because her Republican opponent has the same first and last name as the outgoing Democratic member of Congress she is seeking to replace. In an apparent attempt to exploit potential confusion, Doyle’s website does not mention that he is a Republican.

Aipac’s campaign against Lee is a rematch after it tried and failed to block her during the Democratic primaries earlier this year.

The UDP spent more than $25m in the primaries to defeat candidates it deemed too critical, or insufficiently supportive, of Israel, including about $2.6m against Lee. Most of the candidates opposed by Aipac lost but Lee won her race by a slim margin.

Much of the advertising in support of Aipac-backed candidates in the primaries played up Democratic party values such as equality. One of those opposed by the lobby group, Congressman Andy Levin who lost his primary and seat, on Wednesday tweeted that Aipac’s opposition to Lee revealed its professed support for liberal values to have been a sham.

“If it wasn’t clear before, it certainly is now: AIPAC doesn’t care about our party’s values and priorities and it’s willing to empower extremists and undermine American democracy in order to defeat principled, progressive candidates,” he wrote.

One of those who initiated the letter from members of Pittsburgh’s Jewish community was Ritchie Tabachnick who sits on the steering committee of a more moderate pro-Israel organisation, J Street. Tabachnick said the letter speaks for the majority of the city’s Jews because they are disturbed at Aipac “supporting some of the most extreme Republicans, people who make openly antisemitic remarks promote antisemitic conspiracy theories”.

“It’s quite possible to be pro-Israel and antisemitic. They often go hand in hand. Aipac have chosen to prioritise the-pro Israel and ignore the antisemitic elements that go with it,” he said.

Tabachnick said he believed Aipac was attempting to shut down widening criticism of Israel in the US, a task made more urgent by the expected return of Benjamin Netanyahu as prime minister in coalition with far-right Jewish nationalists.

“They are trying to control the narrative,” he said.

But Tabachnik said he does not believe Aipac represents the views of most of the US’s Jewish community.

“They are a loud, politically smart minority,” he said of the group.

Aipac denies taking sides against the Democrats, saying that Lee’s views put her “outside of the Democratic mainstream”.

Aipac has been approached for comment.