With a crucial vote pending over filibuster rules that would have made strong voting rights legislation feasible, Democratic senator Kyrsten Sinema flew into Houston, Texas, for a fundraiser that drew dozens of fossil fuel chieftains, including Continental Resources chairman Harold Hamm and ConocoPhillips chief executive Ryan Lance.

The event was held on 18 January at the upmarket River Oaks Country Club. One executive told the Guardian that Sinema spoke for about half an hour and informed a mostly Republican crowd that they could “rest assured” she would not back any changes with filibuster rules, reiterating a stance she took several days before during a Senate speech.

The Arizona senator also addressed some energy industry issues according to the executive, who added that overall he was “tremendously impressed”.

The day after the Houston bash, Sinema voted against changing filibuster rules, thereby helping to thwart the voting rights bill.

The Houston gusher of fossil fuel donations for Sinema from many stalwart Republican donors underscores how pivotal she has become, along with West Virginia Democratic senator Joe Manchin, in an evenly divided Senate involving high-stakes battles for Republican and fossil fuel interests.

Campaign finance watchdogs say that the Houston fundraiser reveals much about Sinema’s aggressive efforts to capitalize on her Senate power on matters ranging from climate change to taxes to the filibuster rule.

“Sinema isn’t up for re-election this year, but she’s fundraising full-tilt,” Sheila Krumholz, the executive director of OpenSecrets, told the Guardian. “By her comment to oil-industry attendees last week, she clearly knew her vote to protect the filibuster would please them.”

The Houston fundraiser, which was expected to raise tens of thousands of dollars for the senator’s campaign coffers, offers a stark example of how Sinema has been courting major Republican donors and special interests who, in turn, seem to be increasingly eager to help her.

Sinema’s drive to rope in more big Republican donors was also apparent at a September fundraiser in Dallas at the $18m home of G Brint Ryan, a prominent Republican donor and CEO of a global consulting company, who hosted another money bash last year for Manchin.

Sinema’s stance against changing filibuster rules has also won her support from other top Republican donors such as Stan Hubbard, a Minnesota billionaire broadcaster who gave her $2,900 last September, which reportedly was the first donation he made to a Democrat since 2019.

Hubbard told the Guardian that her opposition to the filibuster was a crucial reason he donated, adding that it would “be terrible to get rid of the filibuster”, and that he thought voting rights were “just fine”, without passing a Democrat-backed bill to protect them.

Little wonder that voting rights advocates were dismayed by Sinema’s staunch opposition to any changes with the filibuster.

“We are very disappointed that Senator Sinema has put formalistic rules over protecting our democracy,” said Danielle Lang, the senior director of voting rights at the nonpartisan Campaign Legal Center.

Sinema’s position on the filibuster rule has sparked anger among liberal backers such as the powerful group Emily’s List, which endorses Democratic women who support abortion rights. One week after Sinema gave a floor speech indicating that she wouldn’t support altering filibuster rules, Emily’s List publicly stated that the group would no longer endorse her.

In her floor speech backing the filibuster rule, Sinema touted the need for more bipartisanship, stressing that she would not “support separate actions that worsen the underlying disease of division infecting our country”.

But Sinema’s vote and speech only spurred more criticism in Arizona where the state Democratic party issued a rare censure in the wake of her continued support for the filibuster.

Arizona’s Democratic party chair Raquel Teran has stated that the vote was a “result of her failure to do whatever it takes to ensure the health of our democracy”.

More broadly, Democratic angst about Sinema was highlighted by a January tracking poll before her filibuster vote that showed just 8% of registered Arizona Democrats had a favorable view of the Senator.

The recent poll reflects a steep drop from the 70% positive rating the Senator had in 2020. Her declining popularity also has been spurred by the senator’s voting against raising the federal minimum wage, and skipping a Senate vote to create a bipartisan commission to investigate the January 6 mob attack on the Capitol by Trump supporters.

Sinema has also drawn brickbats from Democrats for her unwillingness last month to endorse the House passed Build Back Better legislation that she and Manchin were instrumental in whittling down from the measure’s original size, while accelerating their fundraising outreach to rightwing donors and lobbyists.

Sinema told Democratic senators according to the New York Times that she was opposed to any tax increases in personal rates or corporate rates to pay for the bill, which included approximately $550bn for clean energy and climate change measures, a crucial part of President Joe Biden’s agenda.

Leftwing Vermont senator Bernie Sanders was especially irked when both Sinema and Manchin joined all the Senate Republicans in blocking the filibuster rule change, saying that they “forced us to go through five months of discussions which have gotten absolutely nowhere”, and indicating he might support primary challengers to both senators.

Veteran Arizona Republican consultant Chuck Coughlin noted that Sinema “clearly understands the electoral position she is in, and is using this opportunity to raise as much as she can in order to make challenging her a herculean task – whether she runs as a Democrat or an independent.”

Coughlin’s analysis seems on target based on the very robust $4.4m that Sinema’s campaign had in the bank at the end of September.

Charlie Black, a longtime Republican operative and lobbyist, added that “Sinema’s gotten a lot of support from the business community, including both Republicans and Democrats.”

Still, with Democratic attacks on Sinema increasing, the odds are good that if she opts to run again in 2024 she will have a primary opponent, perhaps Congressman Ruben Gallego, who has publicly suggested he might challenge her, and knocked the senator over her filibuster vote.

A group called the Primary Sinema Project that began last summer has raised at least $330,000, including $100,000 during the week after her filibuster speech.

Sinema’s drive to raise big bucks early seems to be underscored by the jump last year in donations from fossil fuel interests, according to campaign finance data.

Last year, Sinema hauled in $24,310 from fossil fuel donors compared with just $7,522 the year before, according to OpenSecrets.

Although there’s no data yet on how much Sinema raised in Houston, a veteran fossil fuel lobbyist told the Guardian that donors at such fundraisers are often asked to pony up the maximum of $5,800 to the senator’s campaign committee, and write another check for as much as $5,000 to the senator’s leadership Pac.

For Krumholz of OpenSecrets, the Houston fundraiser offers a broader message.

“The timing of the fundraiser and Sinema’s filibuster-protecting vote really puts a fine point on the return on investment for her donors.”

Krumholz added that the fossil fuel fundraiser “seems well timed as Congress revisits the $550bn BBB measure focused on climate change provisions, where her vote could help industry minimize new regulatory and tax burdens.”