Since January 6 2021, Liz Cheney has been focused on a single goal: making sure Donald Trump never again occupies the Oval Office.

On Thursday she came a step closer to achieving that ambition as she wrapped up the last in a series of live hearings investigating last year’s attack on the US Congress, and whether the former president played a role in stoking it.

The initial set of broadcasts culminated in a final primetime session on Thursday that explored Trump’s actions as the riot unfolded. “[Trump] refused to defend our nation and our constitution. He refused to do what every American president must,” Cheney told the hearing.

The probe has damaged the former president’s approval ratings, won Cheney an unlikely band of liberal fans and even given the Democrats a bump ahead of November’s midterm elections.

“Of all the people who were going to rescue the Democrats ahead of November, I did not expect it to be Liz Cheney,” said one congressional aide.

“Liz Cheney has been the star of the show,” said Norman Eisen, who worked as an adviser to the committee that conducted Trump’s first impeachment. “It is because of the committee, and especially her, that we now have this weight of evidence in favour of prosecuting Trump.”

The January 6 committee was set up last year following a protracted tussle over which Republicans would serve on it. For Cheney, the decision to reject her party’s leaders and join was an easy one, said people close to her, even though it meant she would go on to lose her position within the Congressional party and might not be able to defend her seat.

“As Liz left Congress on January 6, she was telling fellow Republicans they had to impeach Trump,” said one congressional official who has worked with the committee. “She absolutely believed from the beginning that Trump had incited the riot, and so investigating it was never in question.”

Cheney told The New York Times this week that working on the committee was “maybe the most important thing I’ll ever do”.

Each of the committee members — seven Democrats and two Republicans — has taken the lead for a different part of the investigation. But those who have worked with it say that behind the scenes Cheney was the one making many of the most important decisions.

She broke down the investigation into seven parts, which have each formed the basis for a single hearing. She also sat in on almost all the depositions, even if the committee’s investigators took the lead on questioning. She was heavily influential in two crucial decisions, people briefed on the hearings said: to focus exclusively on the 64 days between the election and January 6, and to let Republicans tell the story.

“This committee has shown you the testimony of dozens of Republican witnesses, those who served President Trump loyally for years,” Cheney said during Thursday’s hearing. “The case against Donald Trump in these hearings is not made by witnesses who were his political enemies. It is instead a series of confessions by Donald Trump’s own appointees, his own friends, his own campaign officials . . . and his own family.”

Cheney’s background as a lawyer meant she approached the material with a rigour that many involved say is rare for a congressional committee. And her impeccable conservative credentials and contacts — she is the daughter of the former vice-president Dick Cheney — meant she was able to secure witnesses others might not have.

“I have worked on a lot of congressional hearings, and have testified live myself,” said Al Schmidt, the former city commissioner of Philadelphia, who testified in the second hearing. “This felt more like being a witness on the stand.”

Richard Donoghue, the former acting deputy attorney-general who testified in the fifth hearing, said: “They have focused on gathering the facts about January 6 and letting the decision makers — whether that is Congress, the justice department or the public — make their own minds up.”

Donoghue added that while several members of the committee participated in his initial deposition — conducted over Zoom in October 2021 — Cheney stayed for almost the entire session and asked the most detailed questions.

Nick Quested, the British documentary maker who was making a film about the far-right groups involved in the violence and who testified during the first hearing, said the committee did extensive background checks on him before deciding he should appear live.

“I could see the incredible detail in their questioning,” he said of the first meeting he held with investigators. “Everything had been rigorously vetted, and confirmed with multiple sources.”

Some of those close to Cheney believe that some of the star witnesses would not have testified had it not been for her. Bill Barr, Trump’s former attorney-general who has provided some of the committee’s most explosive testimony, served in the same cabinet as her father under president George HW Bush.

“There is no way Barr could have got away with saying no to Liz Cheney,” said Barbara Comstock, a former Republican member of Congress and longtime friend of Cheney. “The problem for a lot of these senior Republicans with Liz being there is they could run from the committee but they couldn’t hide.”

The committee’s work has not been universally praised. Some felt members rushed to interview Cassidy Hutchinson, the White House aide who testified that Trump had had an altercation with the Secret Service, only to be contradicted — albeit off the record — by sources at the law enforcement agency.

Others, such as the Republican pollster Frank Luntz, believe the committee has been too selective in its use of video clips, allowing Trump supporters to accuse members of over-editing material.

But polls suggest the hearings have shifted public opinion. Support for prosecuting Trump rose 6 percentage points from late April to late June, with a clear majority now in favour. Meanwhile, Trump’s favourability ratings have dropped from 46 per cent in late April to 42 per cent now.

Democrats who want to turn the midterm elections into another referendum on Trump are delighted.

“I think the return of Maga [Make America Great Again] has started factoring into people’s heads,” said Simon Rosenberg, a Democratic strategist. “This is not going to be your usual set of midterm elections.”

For the Republicans, the hearings have sparked a hunt for a possible alternative to Trump as their 2024 presidential candidate.

Figures released this week show the former president’s fundraising has slowed in the past three months, and more than half of Republican party members say they would vote for someone else in a presidential primary — though they are split between his rivals.

Cheney has hinted that she might launch a presidential run in 2024, but experts say she is unlikely to win a Republican nomination or a general election. Instead, her main achievement might be empowering someone else to take on Trump now that he has been wounded by the committee.

“There is going to be a big field of Republicans running in 2024 thanks to the work of Liz Cheney,” said Comstock. “They should all get to their knees and thank her — although they won’t.”