O'FALLON, Mo. (AP) — A Republican colleague rebuked him on the Senate floor. A home-state newspaper editorial board declared he has "blood on his hands.” But for Josh Hawley, the Missouri senator who staged an Electoral College challenge that became the focus of a violent siege of the U.S. Capitol, the words of his political mentor were the most personal.

“Supporting Josh Hawley ... was the worst decision I’ve ever made in my life," former Missouri Sen. John Danforth told The Associated Press on Thursday. “He has consciously appealed to the worst. He has attempted to drive us apart and he has undermined public belief in our democracy. And that’s great damage.”

Aside from President Donald Trump, who roiled up supporters just before they stormed the Capitol, no politician has been more publicly blamed for Wednesday's unprecedented assault on American democracy than Hawley. The 41-year-old first-term senator, a second-tier player through much of the Trump era, has rapidly emerged as a strident Trump ally, and may be among the most tarnished by the events of Jan. 6 for years to come.

“There will be political fallout for his actions," said Alice Stewart, a Republican strategist and former adviser to Texas Sen. Ted Cruz’s 2016 presidential campaign. “The initial decision to oppose the will of the people was downright wrong. The post-insurrection calculation to continue the charade is fallacious and dangerous.”

Hawley, who defeated Democratic Sen. Claire McCaskill in 2018, was once celebrated by the Republican establishment as a rising star. The Stanford- and Yale- educated lawyer was young, ambitious and savvy. It surprised some when he was first to announce he would endorse false claims of fraud and take up Trump's cause, forcing House and Senate votes that would inevitably fail and in no way alter the election’s outcome.

Support of the challenge to the electoral vote count was seen as keeping in good stead with Trump’s supporters, who dominate the Republican base. The move instantly raised his national profile. Soon Hawley and Texas Sen. Ted Cruz were leading about 10 other senators in the effort — notably not winning over Sens. Ben Sasse of Nebraska or Tom Cotton of Arkansas, two other young Republicans viewed as having presidential ambitions.

As he walked into the Capitol on Wednesday, Hawley cheered on pro-Trump protesters gathering outside the building with a thumbs up and fist pump.

But Hawley's scheme fell apart almost before it got going. As the Senate began debate, pro-Trump mobs barreled into the Capitol and interrupted proceedings. By the time the Senate reconvened, after one woman was shot and killed by police and parts of the Capitol ransacked, support in the Senate for challenging the results had all but evaporated.

Dozens of courts, state elections officials and even Trump's former attorney general have said there was no evidence of widespread election fraud. Still, Hawley asked his Senate colleagues "to address the concerns of so many millions of Americans” by investigating the 2020 vote.

He faced instant rebuke from his own party. With Hawley sitting near, Utah Sen. Mitt Romney blasted those who objected to finalizing President-elect Joe Biden’s election.

Accusing Trump of inciting insurrection, Romney said “those who choose to continue to support his dangerous gambit by objecting to the results of a legitimate democratic election will forever be seen as being complicit in an unprecedented attack against our democracy.”

“That will be their legacy,” he added.

In the deeply divided GOP, that may not be the prevailing view. In Missouri, a state Trump won by almost 16 percentage points, some argued Hawley was blameless.

“For people to blame Sen. Hawley for the people that came up to the Capitol to break the windows — and came wearing helmets and trying to break in — that’s absurd,” said Republican state Rep. Justin Hill, who skipped his own inaugural ceremony Wednesday to attend Trump’s rally in Washington.

Hill, who was the lead sponsor of a Missouri House resolution last month asking Congress to reject some states’ Electoral College votes, said Hawley was “defending the Constitution.”

Yet, GOP state Sen. Shamed Dogan of Ballwin in suburban St. Louis, like Danforth, said late Wednesday he regretted supporting Hawley.

“I have never regretted a vote as much and as quickly as my vote for @HawleyMO in 2018,” he tweeted. “His refusal to accept the legitimacy of Joe Biden’s election, even after today’s violence, is an embarrassment.”

At least one major donor turned on Hawley, now calling him a “political opportunist" and urging the Senate to censure him.

David Humphreys is president and CEO of Tamko Building Products in Joplin, Missouri, who has donated millions of dollars to Hawley and other Republicans. But Humphreys opposed Trump's election in 2016, saying he couldn’t justify supporting Trump to his children.

“I need to say the same about Missouri’s U.S. Sen. Josh Hawley, who has shown his true colors as an anti-democracy populist by supporting Trump’s false claim of a ‘stolen election,’” Humphreys said in the statement late Thursday to the news site the Missouri Independent. “Hawley’s irresponsible, inflammatory, and dangerous tactics have incited violence and further discord across America. And he has now revealed himself as a political opportunist willing to subvert the Constitution and the ideals of the nation he swore to uphold."

Phone and email messages seeking comment from Humphreys on Friday were not immediately returned.

The pile-on continued. The student bar association at the University of Missouri law school, where Hawley taught, issued a statement calling for his resignation.

The Kansas City Star listed Hawley as second only to Trump as responsible for the attack on the Capitol, and noted Hawley had issued a political fundraising solicitation as the siege was underway.

“But this is not about me! It is about the people I serve, and it is about ensuring confidence in our elections," said the email sent just as thousands were marching up Pennsylvania Avenue from a rally outside the White House headlined by Trump. “That’s why I am standing up on behalf of the people I serve to relay their concerns to Washington. For conviction. For principle. For our country. For YOUR VOTE."

Simon & Schuster canceled publication of Hawley's upcoming book, “The Tyranny of Big Tech."

The publisher said it values publishing diverse view points. “At the same time, we take seriously our larger public responsibility as citizens and cannot support Senator Hawley after his role in what became a dangerous threat to our democracy and freedom," it said in a statement.

Hawley responded by calling the decision a “direct assault on the First Amendment.”

“I will fight this cancel culture with everything I have. We'll see you in court."

Danforth, who served for three terms, said he remembers how impressed he was when he first met Hawley at a dinner party when Hawley was just a law student. The young man reminded him of his friend, Democratic Sen. Daniel Patrick Moynihan, Danforth said.

“I felt he could add real intellectual heft and make a great, great contribution to the Senate," he said.

Now Danforth wonders how Hawley will be able to work with his Senate colleagues, even Republicans, moving forward.

“How is he going to operate in the Senate with Republicans? When (Senate Majority Leader) Mitch McConnell pleads don’t do this and he does it, and then this is the consequence," he said. "How is he going to get along with his colleagues? How is he going to do anything? What’s his political future?”

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Beaumont reported from Des Moines, Iowa. Associated Press correspondent David Lieb contributed from Jefferson City, Missouri.