Wisconsin Gov. Scott Walker (R) announced Monday that he was suspending his campaign for the White House, urging other candidates to follow suit. (Reuters)

This story has been updated.

Wisconsin Gov. Scott Walker has suspended his presidential campaign, effectively ending a once-promising GOP presidential bid that collapsed amid tepid debate performances, confusing statements and other missteps.

"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," Walker said in a brief speech in Madison, Wisconsin, on Monday evening. "With this in mind, I will suspend my campaign immediately."

Walker said that because the field is so crowded, candidates have become focused on personal attacks instead of the substantial issues that matter most to voters. He said Republicans have lost the "optimistic view of America" pushed by President Ronald Reagan, Walker's political idol, and urged those still running to "get back to the basics" with a focus on creating jobs, reducing the size of government and strengthening the military.

"To refocus the debate on these types of issues will require leadership," Walker said. "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. This is fundamentally important to the future of the party and, more importantly, to the future of our country."

In making that plea, Walker did not directly name the current front-runner, businessman Donald Trump.

The announcement stunned many of Walker's major supporters, donors, fundraisers and even some of his staff members. Given his tanking poll numbers, many expected dramatic changes to the staff and strategy -- but not such a sudden end.

“I’m stunned and saddened because I think Scott has had a tremendous record of accomplishment,” Fred Malek, a longtime party fundraiser who serves as the Republican Governor's Association’s finance chairman. “He’s a man of the highest character and capacity, and he would have made a great president."

Wisconsin State Assembly Speaker Robin Vos said in a statement: “Governor Walker has an amazing story to tell about turning Wisconsin around. It is unfortunate that the bluster of candidates overshadowed his substance."

When Walker launched his presidential campaign in mid-July, he was considered a top-tier candidate. He was an early favorite in Iowa, where many voters liked that he was a mellow, understated and sometimes boring Midwesterner. And a super PAC supporting his candidacy raised more than $20 million in less than three months.

But as the summer wore on, Walker's campaign quickly became overshadowed by Trump and other candidates who have never held elected office. As Walker slid in early polls, he seemed to struggle to find his place the field, sometimes taking stances or using language that confused some of his longtime supporters. During the first Republican debate, Walker pitched himself as "aggressively normal" but seemed to disappear on the crowded stage. While he tried to be more energetic during the second debate last week, Walker was again overshadowed and hardly spoke during the three-hour faceoff. There were always glimmers of hope within the campaign that the situation would improve, and Walker's campaign was constantly maneuvering -- first targeting Trump's supporters, then trying to tap into anti-establishment sentiments and then, just last week, focusing all of their energy on Iowa.

[How Donald Trump destroyed Scott Walker's presidential chances]

Throughout the summer, Walker made a series of confusing or contradictory comments that often took several days to fully clarify. In August, he seemed to endorse ending birthright citizenship, then said he didn’t have a position on the issue, and then said that he did not want to change the constitution, which many believe guarantees citizenship to those born on U.S. soil. In late August, Walker called building a wall along the Canadian border “a legitimate issue for us to look at,” only to say days later that he never supported the idea and that his words were twisted by the media. Over Labor Day weekend, he refused to say if the United States should accept more Syrian refugees, telling reporters that it was a “hypothetical question” and that he wanted to talk about "reality" – only to say soon after that the United States should not accept more refugees.

Several longtime Walker supporters said they no longer recognized the candidate they had watched rise to national prominence from the Wisconsin governor's office. Walker is best known for aggressively pushing for reforms to the state's public-sector unions in 2011, riling Democrats both in his state and across the country.  He quickly became a favorite of tea-party activists and his calm amid protests at the state capitol landed him on the cover of conservative magazines. He became a regular presence on Fox News. A year later, as he battled and ultimately won a recall election, he was being touted as a possible vice-presidential candidate for GOP nominee Mitt Romney.

“It was nice for him to get that attention in the short run, but it set up expectations he couldn’t hope to maintain,” said Vin Weber, a former Minnesota congressman and adviser to Jeb Bush’s presidential campaign.

Union leadership, which had long considered Walker a top target, reacted quickly Monday to reports that he was suspending his campaign. "Scott Walker is still a disgrace, just no longer national," AFL-CIO president Richard Trumka said in a terse afternoon statement.

In recent weeks, there were clear signs that Walker's campaign was in trouble. His poll results began to resemble a ski slope. And although the super PAC was flush with money, supporters worried that the campaign itself was running low on cash.

The large cadre of staff and paid consultants around Walker have been on what one called a "death watch" for the past several weeks. It was clear to many that a single bad debate performance would spell the beginning of a dramatic downsizing of Walker's campaign, with Walker staffers bracing for spending cuts, layoffs and a shake-up in the campaign leadership. Following last week's debate, the frustration of many fundraisers and major donors exploded, as they demanded that Walker replace his campaign manager, Rick Wiley. Over the weekend, Walker skipped two previously scheduled appearances in Michigan and California, angering Republicans in states with high numbers of delegates, so that he could instead spend more time in Iowa. There, he struck several people as looking exhausted and beaten down.

There aren't many loyal Walker voters in the state left to claim, said Steve Grubbs, Iowa strategist for Republican presidential rival Rand Paul. "The reality is that there was a very significant shift from Walker to Trump over the last 8-10 weeks," he said, adding that it was those voters who might be up for grabs. "As Walker is out, and Trump begins to lose support, those voters will come back into play. And we believe that a lot of those voters are gettable," said Grubbs.

Then came the latest CNN poll on Sunday that was like a punch in the gut: The governor was now polling nationally at less than one percent – so low that he received an asterisk on some charts instead of an actual number.

Still, the candidate kept his deliberations to quit the race very close, with a full schedule of events planned for this week that included campaign stops in Indiana and Virginia and a fundraiser in New York City at the home of one of his major donors, Joe Ricketts. Most staff, including senior aides, found out only Monday that he had decided to suspend his campaign later in the day.

Walker said on Monday that he reflected on the decision at church on Sunday. In suspending his campaign, he thanked everyone who believed in him -- especially his wife, Tonette, and their two sons.

"Most of all, I want to thank God for his abundant grace," Walker said in closing on Monday. "Win or lose, it is more than enough for any of us."

[What happened to Scott Walker?]

Trump -- who has been credited with quickening if not causing the sudden death of Walker's campaign -- praised Walker’s character and gubernatorial record and said he would reach out to his former rival in the coming days to offer encouragement.

“I really liked him a lot," Trump said in an interview with The Washington Post on Monday. "I thought he was a terrific person. He has been a terrific governor. I got to know him pretty well. I’m a little surprised that it hasn’t worked out better for him. Many people thought he’d be the primary competition, at least initially."

Trump, who proudly surrounds himself with a small group of aides, wondered if Walker was hurt by too much advice and management from his political consultants. “He was very loose guy when he came up to see me a few months ago to give me a plaque, but then on the campaign, maybe there were too many people. I think he had too many people, many of them who didn’t know what they were doing,” he said.

Other presidential contenders also offered their praise on Monday evening. In a statement, Sen. Ted Cruz (R-Tex.) called Walker "a good man, a formidable fighter, and an effective reformer." Sen. Marco Rubio (R-Fla.) also used a statement to call Walker "a good man" and "one of the best governors in the country."

Even before Walker announced the suspension of his campaign, rival campaigns had begun contacting top Walker donors to urge them to come aboard. Vin Weber said Bush’s campaign was aggressively reaching out to Walker donors and staffers Monday afternoon. “We thought it happened a little sooner than expected, but it was inevitable. There was not a path back for him, based on his performance as a candidate. And even though he was an asterisk in the polls, his decision will help to clarify the race, sending a strong message to other candidates who aren’t registering to move on and get the party down to 5 or 6 candidates who are viable.”

Gary Marx, a senior adviser to Walker's campaign who coordinated outreach to conservative movement groups, said in an interview Monday that he and others are already looking for work. On Tuesday, Marx said, he will interview with three GOP presidential campaigns, which he declined to name.

A major problem of the Walker campaign, he said, was that it was difficult to generate enthusiasm -- and campaign funding soon dried up. "No matter how much money was in the super PAC, hard dollars still matter," he said. "He didn't have the finances to continue on. Money is ultimately what stops campaigns from going further."

 Philip Rucker, Matea Gold, Sean Sullivan and David Weigel contributed to this report.