WAUKESHA, WI — (CNN) Scott Walker, the political phenomenon who rose to national fame by taking on unions in one of the most blue-collar states, tweeted Monday morning confirming that he’ll seek the Republican nomination for president, just hours ahead of his in-person announcement.

“I’m in. I’m running for president because Americans deserve a leader who will fight and win for them,” Walker tweeted, signing the message with his initials “SW,” indicating it was written by him and not his staff.

About 5,000 attendees had RSVP’d to attend his Monday afternoon event in Waukesha, Wisconsin — a Republican enclave just outside Milwaukee. The rally will be a remarkable political milestone for the 47-year-old second-term governor, who vaulted from the obscurity of the Milwaukee County executive to the top tier of a presidential campaign, thanks in large part to a historic gubernatorial recall effort that nearly ended his career in 2012.

Walker’s rally featured a roster of conservative voices throwing red meat to the crowd of Walker supporters who had been through his heated campaigns and survival of a recall effort. One of the earliest lines to rev up the crowd came from Rachel Campos-Duffy, wife of U.S. Rep. Sean Duffy who took a shot at the former President Bill Clinton.

When she was describing the relationship between Walker and his wife Tonette, Campos-Duffy said “They’ve been married for 24 years. 24 is Bill Clinton’s favorite age.”

In his first foray into a presidential campaign, Walker — one of the most recognizable and polarizing governors in the country — has emerged as a potentially formidable opponent to former Florida Gov. Jeb Bush. Polling has consistently shown Walker leading the pack in Iowa, which borders southern Iowa, but he has fallen off in other national polling following a series of gaffes.

The biggest question for him now, as he becomes the 15th major-name Republican candidate, will be whether he can break out of the pack in other key states like New Hampshire, South Carolina and Nevada.

Walker has been a torchbearer for top conservative priorities, showing again this that he had a few more presents for the party’s rightwing base. Just a day before he was set to kick off his campaign, Walker was signing a $73 billion budget that only narrowly made it to his desk.

Included in there was the elimination of the “prevailing wage” — a minimum wage for construction projects — and a sizable cut to the state’s university system. Walker was forced to backtrack, however, on an unannounced plan to curb government transparency by limiting access to public records.

Indeed, Walker has been a polarizing figure — infuriating and mobilizing the left, in particular organized labor. By Monday afternoon, nearly 100 protesters had gathered outside his rally at the Waukesha Expo Center.

Although Walker waited to launch his White House bid, he has been laying down the groundwork for a national campaign for months. The union-busting governor has been courting donors, traveling overseas and boosting his national profile by publicly tussling with President Barack Obama on issues like the nuclear deal with Iran.
The next several months will be a critical test for Walker. After his campaign announcement, the governor will crisscross the country, including a three-day RV tour through Iowa this coming weekend.

A few months after taking office in 2011, Walker signed a measure to curb collective bargaining rights for most public employees in the state, framing it as an effort to take on the “big government special interests” and give power back to Wisconsin taxpayers.

The move triggered fierce backlash from labor unions and their progressive allies, sparking massive protests at the state capitol in Madison.

Conservatives in Wisconsin and around the country came to Walker’s defense, helping the governor withstand a recall effort in 2012. Walker went on to win re-election by six points in 2014, his third statewide victory in four years.

It is that recall experience, more than any other, that has helped lay the groundwork for Walker’s presidential bid. At the time, Walker said his recall victory proved that “voters really do want leaders who stand up and make the tough decisions.”

That is a message that Walker has promoted in recent months while exploring a presidential campaign, calling on the Republican Party to look for “fresh leadership” and someone with “big, bold ideas and the courage to act on it.

Veteran GOP strategist Kevin Madden, who served as a senior adviser to Mitt Romney’s presidential campaign in 2012, says Walker’s union-busting efforts helped him to burnish his conservative credentials by demonstrating his ability to taken on “the national [Democratic]establishment” and beat them on three consecutive occasions.

Beyond his record as governor, Walker’s Midwestern roots will be an invaluable asset in the GOP nominating fight — Wisconsin voters haven’t picked a Republican for president since 1984.

Walker is poised to make the case that his candidacy could put other Great Lakes states on the electoral map that have been out of reach for Republicans in recent cycles, including Michigan and Pennsylvania, which haven’t voted for the GOP nominee since 1988.

“The path for a Republican to win the presidency comes through the Midwest,” Walker told a crowd in Cedar Rapids, Iowa, in April. “It comes from Iowa and Wisconsin and Michigan and Ohio and we’re even going to include Pennsylvania because they’re part of the Big Ten,” he added.

Walker also has a personal narrative that could enable him to appeal to low-income voters, a group Republicans lost overwhelmingly in 2008 and 2012, with the struggles in the latter campaign fueled in part by Mitt Romney’s “47 percent” comments.

Walker is the son of a Baptist minister and an Eagle Scout, who attended Marquette University for three years before dropping out to take a job with the Red Cross. Walker has taken to citing his humble beginnings during recent appearances, setting up a contrast between his background and that of one of his key 2016 rivals — Bush.

“I realize unlike some out there I didn’t inherit fame or fortune from my family,” Walker said during a speech to a Christian broadcasters convention in February. “I got a bunch of things that were a whole lot better than that. I got from my parents and my grandparents the belief that if you work hard and you play by the rules, here in America you can do and be anything you want.”

Walker has already experienced challenges that come with being thrust into the national spotlight in the months leading up to his campaign’s official launch.

Unlike some of his peers in the Republican field like Bush, who lived through the White House campaigns of his father and brother, or former 2012 presidential candidates Rick Perry or Rick Santorum, Walker is facing head-on for the first time the reality of just how much scrutiny comes with a presidential campaign.

And it’s shown.

Speaking at the Conservative Political Action Conference in February, Walker raised eyebrows when he seemed to compare the task of fighting ISIS to taking on thousands of protesters in his state.

Earlier that month, a trip to London resulted in a slew of unflattering headlines when Walker, despite his best efforts to avoid making news during the overseas trip, punted on a question about the theory of evolution.

In March, several news outlets, including CNN, reported that at a private gathering in New Hampshire, Walker had endorsed a pathway for citizenship for undocumented immigrants. This would have marked a notable reversal for Walker who had staked out a more conservative position on the divisive issue, and pundits were quick to suggest that the governor had flip flopped.

Joe McQuaid, publisher of the New Hampshire Union Leader whose office is a must-visit early state stop for presidential candidates, remarked at the time that the controversy surrounding Walker’s reported immigration remarks showed that this is the governor’s “first time outside of Wisconsin.”
“It is a guy in his first presidential campaign trying to get himself grounded and see where he needs to be nuanced,” McQuaid said in a recent interview. “He hasn’t dealt with these issues on a regular basis.”

Walker could give Bush a run for his money.

Over the past few months, the governor has made aggressive overtures to wealthy financiers and prominent Republican donors, presenting himself as a conservative alternative to others in the field.

Bush’s extensive fundraising network, founded on decades-old family friendships, will be difficult to compete with. But in the earliest stages of the campaign, Walker’s political action committee, Our American Revival, has boasted impressive commitments and donations from prominent donors and bundlers in fundraising epicenters like New York, California and Texas.

And it’s not just deep-pocketed donors that Walker is banking on.

The recall fight that made the governor a national figure could be a boon for his fundraising efforts among small-dollar donors.

Walker raised more than $30 million for the recall campaign, which helped him grow his donor to list to some 300,000 supporters. Walker and his supporters say they’re eager to win over a new generation of donors.

“Our donor is not the tried and true Republican donor in New York City that’s given to everybody since Reagan, Anthony Scaramucci, the founder of the investment firm SkyBridge Capital who is raising money for Walker, told CNN earlier this month. “We don’t have the mercenary donor that’s paying for past political favors.”

The-CNN-Wire ™ & © 2015 Cable News Network, Inc., a Time Warner Company. All rights reserved. (CNN)