MADISON, Wis. (AP) —
    Warning that the Republican presidential race has become too nasty, Scott Walker exited the 2016 campaign on Monday and urged others to quit, too, and "clear the field" so someone can emerge to take down front-runner Donald Trump.
    The announcement marked a dramatic fall for Walker, who was struggling to generate money and enthusiasm after surging into the race's top tier earlier in the year. He will return to his job as governor of Wisconsin, where his term runs through 2018.
    "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," Walker said in a news conference. "I encourage other Republican presidential candidates to consider doing the same so the voters can focus on a limited number of candidates who can offer a positive conservative alternative to the current front-runner."
    Walker said that is "fundamentally important to the future of the party and more importantly to the future of our country."
    One of the last Republicans to enter the race, Walker joined former Texas Gov. Rick Perry as one of the first to leave it. He found himself unable to adjust to Trump's popularity or break out in either of the first two GOP debates. Both candidates warned of the billionaire businessman's influence on the GOP as they stepped aside, although neither called him out by name.
    "Sadly, the debate taking place in the Republican party today is not focused on that optimistic view of America," Walker said. "Instead, it has drifted into personal attacks." Walker's sons, Matt and Alex, attended his speech. They each had taken a semester off from college to campaign with him.
    Anthony Scaramucci , one of Walker's top fundraisers, expressed hope that other struggling candidates will heed Walker's call to distill the field.
    "I think what he did shows real leadership," Scaramucci said. "He's sending a signal to the low single-digiters — the new 1 percenters, if you will — that it's time to go, for the good of the party."
    Walker's departure prompted a good riddance from Richard Trumka, AFL-CIO president, reflecting the hostility between the governor and organized labor.
    "Scott Walker is still a disgrace," Trumka said, "just no longer national."
    Walker's fall was in many ways more dramatic than Perry's.
    He was thought to be a leader in the big pack for much of the year and built a massive national organization, with paid staff spread across the country, that dwarfed many of his rivals in scale and scope.
    "I don't think he made any really big mistakes," said Iowa state Sen. Mark Costello, who endorsed Walker earlier this year. "But people lost enthusiasm."
    Walker, 47, tried to appeal to religious conservatives, tea party conservatives and the more traditional GOP base. He cast himself as an unintimidated conservative fighter who had a record of victories in a state that hasn't voted Republican for president since 1984.
    Like Perry, however, Walker found little room for such a message in a race dominated by Trump.
    Trump tweeted in response to Walker's decision, "he's a very nice person and has a great future."
    Walker came to the race having won election in Wisconsin three times in four years, and having gained a national following among donors and conservatives by successfully pushing his state to strip union bargaining rights from its public workers.
    Walker pointed to those Wisconsin wins, in a state that twice voted for Barack Obama as president, as signs that he could advance a conservative agenda as the GOP's White House nominee.
    He called himself "aggressively normal" and made a splash in January with a well-received speech before religious conservatives in Iowa.
    Groups backing Walker went on to raise $26 million, tapping wealthy donors whom Walker had cultivated in his years as governor and during his successful effort to win a recall election in 2012.
    Walker's primary super PAC, called Unintimidated, had just begun spending for a major push in Iowa — reflecting the governor's last-ditch strategy to place all of his chips on that first-to-vote state.
    The super PAC told federal regulators in a filing Friday that it had spent more than $1.6 million boosting Walker this year, most recently on a $50,000 mailing to Iowa voters. It will now return what it hasn't spent to its donors.
    Many of Walker's troubles were not of Trump's making.
    He took days to clarify whether he supported ending birthright citizenship. He initially showed interest in building a wall between the U.S. and Canada, only to laugh it off as ridiculous. He also declared he wasn't a career politician, despite having held public office for 22 straight years.
    After his fade in polls, Walker took a more aggressive approach, promising to "wreak havoc" on Washington. He vowed to take on unions as president, just as he did as governor, outlawing them for federal government workers.
    But the anti-union policy proposal fell flat; announced in the days before the second GOP debate, it wasn't mentioned at all — by Walker or anyone else — on stage.
    While only Texas Sen. Ted Cruz and former Florida Gov. Jeb Bush had more super PAC money available to boost their chances in the original 17-person 2016 Republican field, Walker struggled to generate money for his official campaign.
    He has yet to report fundraising totals to federal regulators, but top fundraisers and donors have said his plummeting poll numbers left them struggling to generate cash.
    Walker called his senior staff to the governor's mansion in Madison on Monday to review recent polling, in which he was mired at the bottom, and his campaign's finances.
    "I'm disappointed," said Stanley Hubbard, a billionaire media mogul from Minneapolis who had backed Walker's campaign. "He's a good man and would have been a good president."
    As word spread of his decision to exit the race, Republican operatives in Iowa working for other campaigns were already making plans to contact state lawmakers who had committed to support Walker.
    Walker had assembled a campaign organization in every one of Iowa's 99 counties and had a number of state lawmakers committed to him.
    Cliff Hurst, one of Walker's New Hampshire co-chairs, was already planning to shift to Florida Sen. Marco Rubio's campaign.
    He said he knew about three days ago that "it was over" and had been discussing an endorsement of Rubio as of Monday morning, before Walker's announcement became official.