Of the three, Walker — who officially kicked off his presidential bid Monday — may be the least well known to the American public.
But his high-profile battles against the unions and, most significantly, his survival of a recall vote, have made him a folk hero among the GOP and a political force to be reckoned with.
"He became a cause celebre for a lot of conservatives around the country," said political analyst Geoffrey Skelley, associate editor of Sabato's Crystal Ball at the University of Virginia Center for Politics.
Politics is all about fundraising, and Walker, 47, attracts a lot of donor interest. An analysis conducted by the New York Times revealed that almost half of the top 250 Republican donors gave money to Walker's political campaign to become governor.
That's more than to the respective campaigns of Rubio (30 per cent), Texas Senator Ted Cruz (20 per cent) and New Jersey Governor Chris Christie (10 per cent). (Bush wasn't evaluated, as his last campaign was in 2002 when the network of top conservative fund-raisers was quite different, the Times noted.)
"A lot of people don't know who he is yet, but a lot of people who do know him among Republicans seem to like him and he has fairly broad support. He probably has a lot of room to grow," Skelley said.
"He will have the money support to make sure they do know who he is."
College dropout a polarizing figure
That broad support, which includes independent voters, has enabled Walker, a staunch social conservative, to be elected governor of a blue Democratic state three times in four years, even though he's been a polarizing figure.
"His supporters absolutely adore him and his opponents absolutely loathe him," said Patrick Marley, political reporter for the Milwaukee Journal Sentinel.
Walker attended Wisconsin's Marquette University, but never graduated, instead, according to his bio, leaving to work full-time in financial development for the American Red Cross. If Walker were to be elected president, he would be the first college dropout to do so since Harry Truman.
Walker made his first run for governor in 2006, but quit before the primary was held. He then spent the next four years holding small meetings with voters across the state, building support and honing his skills as a retail politician, Marley said.
"He is someone who has shown he can be very committed talking to a room full of 10 or 15 voters or an event with thousands of people," said Marley, who also co-authored the book More Than They Bargained For: Scott Walker, Unions and the Fight for Wisconsin.
Walker won the governorship in 2010, running on a platform of fiscal prudency that included a pledge to make public workers pay more for their benefits.
But once he came into office he went a step further, taking away their core ability to collective bargaining, something he had not discussed during the campaign.
The reaction from the unions, unsurprisingly, was immediate and overwhelming, sparking some of the largest protests in Madison since the Vietnam era, Marley said.
Most significantly, his actions prompted his political opponents to instigate a recall vote, meaning Walker would face voters again only two years after first being elected.
But that recall also made him a hero to many Republicans dismayed that such measures, normally reserved for misconduct or malfeasance while in office, would be used against him.
"And the fact he won, and won by a bigger margin in 2012 than he did in 2010 – it became this almost folk tale to Republicans that you can take big and bold action and win," Marley said.
It also allowed Walker, as a sitting governor of a relatively small state, to develop a national fundraising base – allowing him to draw from the average person who makes a $25 or $50 donation, as well as from some of the richest donors in the country, Marley said.
While his strong opposition to same-sex marriage and abortion has endeared him to social conservatives, Walker has been able to appeal to the middle by focussing more on the economy and taxes.
But he has also attracted negative press for some of the statements he made. He was dogged for days after speaking to a London, England-based think-tank in February when he refused to say whether he believed in the theory of evolution.
Then, in later interview at the National Governors Association in Washington, he said he didn't know if U.S. President Barack Obama is a Christian.
On immigration, he's been all over the map. Walker was initially in support of a pathway to citizenship for illegal immigrants, for example, but he reversed that stand, going so far as to suggest instead that in terms of legal immigration policy, American workers and wages need to be protected first.
Those comments angered some on the right, including the right-leaning Wall Street Journal, which slammed him for what it considered were protectionist ideals.
Walker will boast that his blue state credentials make him the most electable. But whether he can deliver, even his own state, is another matter. The Democratic candidate for president has won Wisconsin every year since 1988.
And his appeal hasn't yet been tested on the national stage.
"We've never seen him on the ballot when you can have that kind of turnout," Marley said. "I don't know if he would perform any better than any other Republicans."Suggest a correction