Pro-Israel lobby groups have poured millions of dollars into a Democratic primary for a Maryland congressional seat on Tuesday, in the latest attempt to block an establishment candidate who expressed support for the Palestinians.

A surge in political spending by organisations funded by hardline supporters of Israel, led by the American Israel Public Affairs Committee (Aipac), has reshaped Democratic primaries over recent months even though debate about the country rarely figures as a major issue in the elections.

Critics accuse Aipac and its allies of distorting Democratic politics in part because much of the money used to influence primary races comes from billionaire Republicans.

Aipac has spent $6m on Tuesday’s contest in Maryland, more than any other organisation, to oppose Donna Edwards, who served eight years as the first Black woman elected to Congress from Maryland before losing a bid for the Senate in 2016.

Edwards is endorsed by the House speaker, Nancy Pelosi, among other leading Democrats.

But she angered some pro-Israel groups during her stint as a representative by failing to back resolutions in support of Israel over its 2011 war in Gaza and other positions. She also backed the Obama administration’s nuclear deal with Iran when it was strongly opposed by the Israeli government and therefore Aipac.

Aipac launched a super political action committee, or super Pac, the United Democracy Project (UDP), in December as a legal mechanism to spend unlimited amounts to directly influence elections and counter growing criticism within the Democratic party of Israel’s continued domination of the Palestinians.

The lobby group kickstarted the UDP with $8.5m and donations from wealthy donors with close ties to Israel. They include two Republican billionaire businessmen and Trump campaign funders, Paul Singer and Bernie Marcus, as well as the billionaire Israeli American Democratic donor Haim Saban.

UDP-funded television ads criticising Edwards make no mention of Israel and instead attack her as an ineffective politician who got nothing done during her stint in Congress. Over the past two months, Edwards has lost a significant lead in opinion polls over her rival, Glenn Ivey, who is now marginally ahead.

A more liberal pro-Israel group, J Street, which calls for the US government to take a harder line with the Israeli government to end to the occupation, has backed Edwards through its Super Pac with about $700,000 in ads.

A J Street spokesperson, Logan Bayroff, accused Aipac of being a Republican front organisation in part because of its endorsement of members of Congress who voted to overturn Joe Biden’s election victory following the 6 January 2021 storming of the Capitol.

“It’s alarming that a group that has endorsed some of the most rightwing extremist Republicans, with a super Pac funded in part by Republican billionaire megadonors, could go into a Democratic primary and spend and spend with the single-minded purpose of crushing a fairly popular mainstream candidate who they’ve labelled anti-Israel with no evidence, no real justification at all, for such a claim,” Bayroff said.

“This is all about trying to drive the party back into more rightward direction on Israel and foreign policy. It’s really alarming and it’s fundamentally anti-democratic when a group can influence this process in such a way because most voters wouldn’t know where this money is coming from. I think that’s dangerous.”

Bayroff said Aipac and UDP were attempting to intimidate candidates into “feeling that they cannot offer good faith criticism of Israeli policy, that they cannot vocally support Palestinian rights”.

“They recognise that the political space on these issues in the Democratic party has opened up and they want to try to arrest and reverse that trend, and push us back to a place in which there’s really very little public debate or discussion about the correct American role in the region,” he said.

The most visible sign of that shift has come from members of “the Squad” of congressional progressives – Alexandria Ocasio-Cortez, Ilhan Omar and Rashida Tlaib – who are unusually vocal in their support of the Palestinian cause. Opinion polls also show that younger Democrats, including American Jews, are more openly critical of Israel.

A UDP spokesman, Patrick Dorton, dismissed J Street’s accusations including the charge of being a Republican front group by pointing to Saban’s financial support.

“We’re exercising our democratic first amendment rights in participating in these elections. If you want to look at politicians who’ve intimidated people and chilled discussion on the US Israel relationship, look at the Squad,” he said.

“In part UDP was formed because there were an increasing number of candidates with radical anti-Israel views running for Congress. Our view is that is dangerous for American democracy and could negatively impact the the bipartisan support for the US-Israel relationship.”

UDP and other pro-Israel groups, such as the Democratic Majority for Israel and Pro-Israel America, have spent heavily to oppose candidates regarded as anti-Israel in Democratic primaries from Texas to Ohio and California.

The UDP helped defeat six of seven contenders it opposed, including the co-chair of Bernie Sanders’ most recent presidential campaign, Nina Turner, in a solid Democratic seat in Ohio. Turner, who has argued that substantial US aid to Israel should not be used to perpetuate the occupation of Palestinian land, at one point held a lead of 30 percentage points – but lost.