For months, Gov. Scott Walker sat atop political oddsmaker Larry Sabato’s list of likely presidential contenders.

Walker topped national and key contest polls for much of 2015, but eventually he was tripped up by a GOP electorate distrustful of elected officials, the rise of candidates from the private sector — Donald Trump, Ben Carson and Carly Fiorina — and Walker’s own repeated blunders when he went off script, said Sabato, director of the University of Virginia Center for Politics.

“Every cycle there is one candidate who looks good on paper, and then they just don’t do it,” Sabato said Monday after Walker announced he was ending his campaign.

Starting with his comparison of pro-union Wisconsin protesters to Islamic State terrorists, and continuing with novel and changing positions on immigration, Walker kept demonstrating he hadn’t done his homework and couldn’t speak well off the cuff, Sabato said.

“I thought he had more skill than he apparently does,” Sabato said. “And I guess the other part of it is he turned out not to be very good at putting policy together on the fly, which you have to do in a presidential campaign. You have to respond to questions you didn’t expect, and he was always going off track.”

Walker was mocked for suggesting a wall to stop illegal immigration from Canada was worth considering, and he revised his stance on birthright citizenship for children of immigrants three times in a week as he tried to match the tough talk of Trump, said Charles Franklin, director of the Marquette Law School Poll.

“The campaign had trouble finding a message and sticking with it,” Franklin said. “That’s unusual for Scott Walker, with what we’ve have seen in his gubernatorial campaigns, which is one message hammered over and over and over.”

Early on, Walker was attractive to both establishment and tea party Republicans because of his record of winning elections in a purple state while imposing hard-line conservative policies, said Amy Walter, national editor of the Washington-based Cook Political Report.

But when he lurched toward more conservative positions on gay marriage, abortion and immigration, he raised questions about who he really was, Walter said.

“Instead of being the candidate who was leading, he looked like the candidate who was following, positioning himself around the issue of the day,” Walter said.

Veteran GOP strategist Liz Mair, who was on Walker’s campaign staff briefly, said on her Twitter feed Monday that she was sad to see him crash because, she said, he could have been a good president.

“It’s really, really easy to make major missteps and be bitten in the butt by them,” Mair said. “But he could be back, and improved, in 4 (years).”

Walker hired staff who built a massive operation that was unsustainable when donor money started to dry up, and he was overconfident, Mair said. She added other mistakes were that he was prone to “instinctively answering ‘yes’ and ‘absolutely’ to things (and) comparing lots of things to (his) union fight.”

Things he got wrong, according to Mair: “Misunderstanding the GOP base, its priorities and stances. Pandering. Flip-flopping. Treating Iowa as locked down, boasting early of the ability to win even in states like Nevada where winning always looked improbable.”

Former Wisconsin GOP Gov. Tommy Thompson said Walker likely had several reasons for dropping out, and he pointed to a recent CNN/ORC national poll showing Walker registering at less than 1 percent nationally.

“When your polls get out, your money dries up,” said Thompson, who ran for president in 2008 and withdrew before primaries began. Thompson said he was proud of Walker.

“The state of Wisconsin should be proud of the campaign that he ran — he had Wisconsin out there front and center,” Thompson said.

Among Walker’s problems, in hindsight, may have been his promise not to officially announce his candidacy until the state budget was finalized, said Franklin, the Marquette poll director.

Disagreements between Walker and the Republican-dominated Legislature over debt levels and other matters prolonged the budget debate. Walker didn’t sign the spending plan until July 12.

By then, other strong candidates had entered the race. And while Walker’s political action committees had raised large amounts of money for advertising, his campaign couldn’t legally start corralling donors until his candidacy was official.

Walker’s poll numbers rose immediately after his July 13 announcement in Waukesha, but they soon started wobbling, which likely made it difficult to raise money his campaign needed to cover the costs of a large staff and travel expenses, Franklin said.

Reporter Molly Beck contributed to this report.