Indigenous leaders who mobilized Arizona voters to help elect Senator Kyrsten Sinema say the centrist Democratic lawmaker can no longer count on their support, as communities feel betrayed.

Sinema was elected in 2018, halfway through Trump’s term, amid growing alarm at the rollback of voting rights and environmental protections that disproportionately affect tribal communities. Sinema entered the Senate after six years in Congress, having beaten her Republican opponent with 50% of the vote, thanks in part to the large turnout among Indigenous Americans in Arizona.

Two years later, another record turnout among the state’s 22 tribes was crucial in flipping Arizona from red to blue and securing Biden’s path to the White House.

But Sinema’s record as a senator is under mounting scrutiny after she and West Virginia Democrat Joe Manchin sided with Republicans to keep the Senate filibuster rule in place, effectively killing legislation designed to counter an onslaught of voting rights restrictions in states across the country.

The sweeping voting rights reforms include the Native American Voting Rights Act (Navra) , which would allow tribes to determine the number and location of voter registration sites, polling places and ballot drop boxes on their reservations.

Navra is widely supported by tribal nations, but if the filibuster remains intact Democrats need 10 Republicans to vote in favor of the bill.

Sinema supports the voting rights legislation, but it has no chance of passing unless the filibuster – the Senate tradition designed to allow the minority party to prolong debate and delay or prevent a vote – is removed.

Jonathan Nez, president of the Navajo nation, the largest tribe in the US, told the Guardian: “We’re disappointed in the senator. She’s on the national stage due to the Native American vote and large turnout among Navajo people. If she doesn’t deliver on what’s important to us, I’m sure there’s another candidate who will.”

Nez added: “There is much frustration among voters, the Native American voting rights act is very important to us given the way the state of Arizona has been chipping away at our voting rights.”

Nationally, there are 574 federally recognized American Indian tribes and Alaska Native Villages, accounting for about 1.5% (4.5m) of the total population.

This includes 22 tribes in Arizona, including the Tohono O’odham nation, the Hopi tribe, three Apache tribes and the Navajo nation, whose territory extends into Utah and New Mexico. Just over 27% of Arizona is governed by tribal sovereignty.

The political culture varies from tribe to tribe – and some in Arizona have publicly voiced support for Sinema – but those in Indigenous communities are less likely to be registered to vote than other groups due to a number of historical barriers. Indigenous Americans did not have the right to vote until 1948. Longstanding structural obstacles, including poor roads, scarce public transit options and limited postal and translation services, have restricted their participation in elections.

In recent years, Arizona and several other states have passed laws and regulations that make voting even harder for Indigenous Americans – and other underserved communities who tend to vote Democrat. This includes restrictions on early-voter locations, mail-in ballots and ballot collection drives, which legal experts say disproportionately affect ruralIndigenous voters.

The US Senate has also thwarted two bills Republicans had previously blocked four times with a filibuster – the Freedom to Vote Act and the John Lewis Voting Rights Advancement Act.

Navra, which does have some bipartisan support, would ban states from closing or merging polling sites on reservations without tribal consent, and would require states with voter identification laws to accept tribal ID.

Sinema’s support of the filibuster has undermined a mammoth effort by grassroots activists to mobilize Indigenous voters, according to Tara Benally, field director of the Rural Utah Project, a nonprofit that mobilizes underrepresented voters in rural Arizona and Utah.

Benally said: “Our community outreach workers spent hours upon hours registering and gaining the trust of voters in the run up to the 2018 and 2020 elections after decades of being pushed aside at the local, state and federal level.

“Senator Sinema has completely undermined what we did and betrayed her voters. I highly doubt people will vote for her again unless by some miracle she changes her values and stops working against those who elected her.”

About 10,000 new Indigenous voters were registeredin Arizona ahead of the last presidential election – which Biden won by just over 10,000 votes. In 2020, voter turnout on the reservations ranged from 41 to 71% compared to 29 to 57% in 2016.

Torey Dolan, an Indigenous fellow at the Indian Law Clinic at Arizona State University, said: “Sinema got the lion’s share of the votes on the reservations; they played a significant role in getting her and Biden elected. Her opposition to removing the filibuster means a death knell for Navra, and that has left a sour taste in a lot of people’s mouths here.”

She added: “[Going forward] it will be hard to encourage people to register when they don’t see their votes fulfilling the hopes and promises placed on them.”

Sinema voted with Donald Trump 63% of the time as a member of the House (2016 to 2018) and 26% of the time as a senator (2019 to 2021), according to FiveThirtyEight. Voting for the filibuster was the first time she voted against her party since Biden took office.

In addition to voting rights, Biden’s $1.75tn sweeping economic recovery and social welfare spending initiative is also in serious trouble.

The Build Back Better (BBB) bill includes funding for affordable housing, healthcare, nutrition, education grants and child support – basic services which are disproportionately lacking in tribal communities due to the US government’s failure to comply with treaty obligations. Unlike towns and cities, tribes cannot raise money through property taxes as reservation lands are held in trust by the federal government.

The BBB also includes $550bn to tackle greenhouse emissions, the country’s largest ever climate crisis investment. It builds on other major infrastructure investments, such as the American Rescue Plan, that have benefited tribal communities.

Unlike Manchin, who outright opposes the bill, Sinema has signaled support for the White House’s BBB framework but has not committed to voting for it in its current form.

Hannah Hurley, Sinema’s spokesperson, said: “Kyrsten remains laser-focused on delivering lasting solutions for tribal communities across Arizona, and thanks to strong partnerships with tribal leaders she’s delivered historic investments improving tribal roads and bridges, ensuring cleaner water, deploying broadband, helping tribes tackle climate change challenges, and strengthening health care resources. She will continue to work with tribal communities in Arizona expanding economic opportunities and ensuring the federal government honors its obligations.”

But Eric Descheenie, former Arizona state house representative for the district which includes the Navajo nation, said Sinema faces a major trust issue.

“The climate change agenda needs help from senators like Kyrsten Sinema, who represents some of the most influential tribes in the land, and needs to do a better job heeding the wisdom of our people … The problem is, no progressive can trust her,” Descheenie said.