Scott Walker announced Monday that he is withdrawing from the 2016 GOP presidential race — a stunning collapse for the Wisconsin governor who was billed as an early favorite to be the party’s standard-bearer, only to have had his hopes dashed in a matter of months.

Speaking in Madison, Mr. Walker said he is stepping out of the race to help winnow the field and said he hopes voters will rally behind a conservative alternative to front-runner Donald Trump.

“Today I believe that I am being called to lead by helping to clear the field in this race so that a positive conservative message can rise to the top of the field,” Mr. Walker said. “With this in mind, I will suspend my campaign immediately.

“I encourage other Republican presidential candidates to consider doing the same so that the voters can focus on a limited number of candidates who can offer a positive conservative alternative to the current front-runner,” he said, alluding to Mr. Trump.

Mr. Walker’s rise on the national scene, and his precipitous departure in just over two months, is the latest chapter in a political thriller that is rewriting the conventional wisdom of the 2016 election.

Outsiders — no matter how brash, inexperienced or corporate-tarnished — are the rage. And traditional politicians are finding their governing records and their fundraising prowess gain little traction with an electorate eager for radical change.

The dynamic could further shrink the field in coming weeks, touching off an unusual battle for the heart and soul of the Republican Party, where business acumen and authenticity play larger than likability and traditional political skills.

Impact in Iowa

For now Mr. Walker’s announcement, coupled with former Texas Gov. Rick Perry’s exit just over a week ago, opens up a pool of support and could help his rivals in early-nominating states, including Iowa, Wisconsin’s western neighbor.

The son of a Baptist preacher, Mr. Walker was thought to be a good fit for the evangelical and born-again Christians who play a pivotal role in the Iowa caucuses that kick off the nomination race.

“This is good news for candidates running in Iowa’s economic conservative lane,” said Matt Strawn, a former Iowa GOP chairman. “With Walker’s and Perry’s exits, it’s easier to put together a plurality on caucus night among those voters. Now [the] question is which candidate will seize on this opportunity in Iowa.”

“The anti-establishment and evangelical conservative lanes in Iowa remain as congested with candidates as ever, and a splintering of those votes provides a great opportunity for a candidate coming from Iowa’s economic-centric block of Republican voters,” he said.

The announcement also could be a warning sign for others.

“I think that once Gov. Walker got pinned with a ‘front-runner’ label, the campaign struggled to match those expectations, and then some lower polling cascaded into diminishing contributions. With this crowded a field, it is easy to have this happen,” said Steve Duprey, a New Hampshire Republican National Committee member.

“Memo to candidates: Front-runner status can be the kiss of death for a campaign if it occurs too early,” he concluded.

An official of a national conservative organization feared Mr. Walker’s sudden departure would boost the chances that a moderate in conservative clothing would wind up as the 2016 nominee.

“It is unfortunate to have two solid conservatives, Walker and Perry, remove their voices this early in the decision process,” said Willes K. Lee, executive vice president of the National Federation of Republican Assemblies. “Republican voters must continue to hear conservative solutions to avoid repeating the awful 2008 and 2012 establishment mistake.”

Union showdown

Mr. Walker emerged as a rising star within the party after winning fierce showdowns with public sector unions in his home state. In 2012, he became the first governor in U.S. history to survive a recall election, and he won re-election in 2014 — his third statewide victory in four years.

That gave him little time to gear up for a presidential run.

Still, Mr. Walker’s supporters believed he could bridge the gap between the Republican establishment and the grass-roots conservatives that have fought over the direction of the party.

But the 47-year-old struggled under the national spotlight.

He sent mixed message on issues, particularly on immigration, stumbling through his recent response to whether he agreed with Mr. Trump’s calls to end birthright citizenship.

Craig Robinson, a former GOP operative who runs the Iowa Republican website, said Mr. Walker’s waffling on issues undercut the image he had built over the years during his battles with unions.

“It completely eroded how you viewed Scott Walker coming into the race as a strong conservative who stands on principles. In the campaign, he was exactly not that,” Mr. Robinson said. “So over time I think it eroded the support he had in the early polls.”

He added, “I think it is a campaign of missed opportunities and self-imposed errors.”

Poll collapse

A CNN/ORC poll released this week showed that Mr. Walker had been reduced to an asterisk because he did not even get support from 1 percent of respondents. He joined such candidates as former Virginia Gov. Jim Gilmore, Louisiana Gov. Bobby Jindal, Sen. Lindsey Graham of South Carolina and former New York Gov. George Pataki — none of whom have qualified for either of the two “prime-time” presidential primary debates.

Liz Mair, a former top aide to Mr. Walker’s political action committee, ripped apart the campaign via Twitter, saying that Mr. Walker shot himself in the foot by pandering and flip-flopping on issues and losing “sight of his real identity as a political leader.”

Ms. Mair, who resigned from the campaign after criticizing Iowa’s “front-running status,” said Mr. Walker failed to educate himself on the issues and hired outsiders who “spent a lot to build out a massive operation that would not be sustainable unless financing remained amazing forever.”

She also hinted that there was more to the story. “Word is he just avoided getting tied to a very bad story that could well have been coming,” she said.

Kyle Kondik of the University of Virginia’s Center for Politics said Mr. Walker’s fall was “very reminiscent” of former Minnesota Gov. Tim Pawlenty’s campaign in 2012, which also ran out of steam in the early innings of the race.

“Walker started the cycle with a chance to become a consensus insider-outsider candidate,” he said. “He spent the first half of 2015 alienating the party establishment by tacking to the right and making mistakes. Then came the rise of Trump, and he lost his anti-establishment foothold too.”

Fundraising woes

Fred Malek, who served as national finance co-chairman of John McCain’s 2008 presidential campaign and as finance chairman of the Republican Governors Association, said Mr. Walker’s fundraising — much like Mr. Pawlenty’s in 2012 — had likely dried up, making it hard to keep his official campaign running.

“Tim woke up after the straw poll in Iowa, and he discovered that they were $400,000 in debt, and they had lost the Iowa Straw Poll, and it would be harder to raise money. So, therefore, it is very similar here,” Mr. Malek said.

“This time there was no straw poll, but it was a series of polls that made it difficult for him to raise money, and my guess is he finds himself in campaign debt and he didn’t feel it was possible to raise the money to sustain the level of spending,” he concluded.

Some Republicans said Mr. Walker can take some solace in the fact that he is following in the footsteps of George H.W. Bush, Bob Dole, Ronald Reagan, John McCain and Mitt Romney — past GOP presidential nominees that lost the nomination race at least once.

“Scott Walker is a young man,” said Jeff Kaufmann, Iowa GOP chairman. “We are not saying goodbye to Scott Walker in the political arena. The man has to much to give.”

Mr. Walker’s rivals, meanwhile, showered him with praise.

“Scott Walker is a good man who has a proven record of fighting for conservative reforms,” former Florida Gov. Jeb Bush said via Twitter. “I know he’ll continue to do that as Governor.”

Sen. Ted Cruz of Texas called Mr. Walker “a formidable fighter and an effective reformer,” while Sen. Marco Rubio of Florida said Mr. Walker “remains one of the best governors in the country, and I have no doubt that he’ll continue the fight for conservative principles.”

⦁ Ralph Z. Hallow contributed to this report.