Marco Rubio and Scott Walker are both 40-something, fast-rising Republican stars who are building presidential campaigns on the notion that a fresh-faced candidate from humble beginnings is best equipped to defeat Hillary Clinton —and, before her, Jeb Bush.

So well positioned, in fact, that Walker floated the idea that they both might be on the GOP ticket as the presidential and vice presidential nominees come November 2016.

“I've actually had quite a few people — grassroots supporters, donors, and others — who have made that suggestion,” Walker told Bloomberg News on Thursday. “We'd just probably have to arm-wrestle over who would be at the top of the ticket.”

Surely, Walker is getting a bit ahead of himself here. There are still months -- and many, many competitors -- to go before the first caucus and primary. Walker isn’t even an official candidate yet, let alone the presumptive nominee. But the relationship between Wisconsin’s second-term governor and Florida’s junior senator bears watching over the next several months, as their common goals but nuanced paths could make them the Frenemies of 2016.

The two figure to become each other’s top rival, but for now, they have one shared obstacle in their path that they will be eyeing to overcome: former Florida Gov. Jeb Bush.

Bush sarcastically joked at a New Hampshire confab packed with likely Republican presidential candidates earlier this year that he — meaning his family name and the financial and political network that comes with it — was scaring off the competition. Bush’s self-deprecating quip is proving more apropos with each passing week. Ohio Gov. John Kasich acknowledged as much last week, saying he is considering a presidential bid because Bush hasn’t taken the oxygen out of the room, as had been anticipated.

After a six-month exploratory phase, Bush is set to formally launch his presidential bid in Miami on Monday amid stories about the weaknesses of his candidacy, a drop in poll numbers, and a shake-up of his staff. Meanwhile, Rubio and Walker are gaining attention and building momentum, unafraid to take swipes at Bush for his family fame or fortune as they go after Clinton.

Both are hoping to emerge as the candidate acceptable simultaneously to the moderate, establishment wing of the party and it’s more conservative Tea Party base.

Walker plays up his experience as a conservative governor who took on public employee unions in his blue state and won at the ballot box. He hopes his Midwestern appeal will pay off electorally but also politically, presenting himself as an unpretentious middle-class guy who shops at Kohl’s and rides Harley-Davidson motorcycles. Walker has taken more conservative stances while running for president, especially on immigration, gay marriage, and abortion – positions that could turn off more moderate Republicans and, potentially, cut into his electability argument.

“As Walker flexes his conservative muscle … he also risks being easily portrayed as 'out of touch' to the moderate voters needed to win next fall,” writes Amy Walter of the Cook Political Report.

Rubio’s sponsoring of a comprehensive immigration reform bill, which he later distanced himself from, could present some challenges among the conservative GOP base. Rubio has attempted to navigate the tricky waters of this issue by arguing for a border-security-first approach, while also talking about some kind of eventual path to citizenship for those in the country illegally. But Rubio has also turned some heads by agreeing to appear at the Family Leadership Summit in Iowa next month, which attracts the most conservative, evangelical-aligned activists in the state.

“The candidates, and the voters themselves, are still trying to figure out which one of them can beat Bush, and be an alternative to Bush,” says Jamie Burnett, a New Hampshire Republican consultant. “The question is, how [do] Rubio or Walker distinguish themselves from one another to take out Bush and win the nomination?”

While Walker and Rubio are making similar generational-contrast pitches, their backgrounds, current jobs, and paths forward are different, which will help determine which emerges as the more viable alternative — if at all.

For starters, their rivalry can be considered something of a proxy fight in the debate over which public office, governor or senator, is a more suitable a requisite for president, especially in an election cycle where both economic and foreign policy experience are of interest to the public.

Walker has played up his economic and executive experience and Rubio is positioning himself as the candidate best versed in foreign policy.

The Florida senator told the Des Moines Register in an April interview that governors “can certainly read about foreign policy, and take briefings and meet with experts, but there is no way they'll be ready on Day One to manage U.S. foreign policy."

Walker responded during a campaign stop in Iowa that same month by saying Rubio’s logic undermined the leadership abilities of Ronald Reagan, who served two terms as California governor before he became president, but never held office in Congress at all.

"I think governors innately have the ability to lead,” said Walker. “We are every day required to use our Cabinet to make decisions, not just give speeches, not to just travel to foreign places but to ultimately make decisions based on using top talent in our Cabinet and our management team.”

Beyond current positions, each is positioning himself in primary field differently. While Walker’s blue state credentials could make him acceptable to moderate Republicans, he is also tapping more into a conservative activist, and largely white, base that likes his Wisconsin record as a fighter.

Rubio, from the diverse and electorally rich Sunshine State, is appealing as a transformational candidate, one who advisers say has potential to reach beyond the GOP and appeal to independents and moderate-leaning Democrats. This week, for example, Rubio’s campaign sought to turn questions about his personal finances into a story line portraying him as an ordinary fellow with working-class values. And as a Cuban-American with a compelling biography, Rubio can wage a history-making campaign of his own to compete with Clinton’s.

Combined, their biographies make Walker and Rubio stronger together than individually, which is why they are often mentioned together when early-state voters run through their list of preferred candidates. “Walker's and Rubio’s strength lies, in large part, because they are each other's number two,” says one Iowa Republican operative. “More often than not, those two names are mentioned in tandem.”