Photo by AP Images

With 21 candidates officially running for president—and 16 from the GOP alone—2016 promises to be one of the most crowded fields in history.

Take a closer look at the polls, election fundamentals and betting markets, however, and there are really only five viable candidates who can ultimately win the presidency. Let’s break down the true contenders, complete with the biggest hurdle and most distracting red herring for each.

Hillary Clinton – The Default Democrat

Hillary Clinton Presidential Candidate Profile | InsideGov

If you want to question Hillary’s viability, you’ll probably make one of two arguments. The first is that Democrats will rally behind a Hillary alternative, like Bernie Sanders. The second is that Clinton will cruise to the party nomination but lose in the general election.

Of the two arguments against Hillary, the first one isn’t very strong. While she did lose to a rising Barack Obama after seeming unbeatable in 2007, her current numbers are much stronger than before. Even with a recent boost for Sanders, Clinton holds a 40-point lead in the polls (55% for Clinton, 15% for Sanders). Back in July 2007, she was only ahead of Obama by a margin of 10 points (35% to 25%). When you factor in Clinton’s huge lead in endorsements, she is almost certain to win the Democratic nomination.

Turn to the general election, however, and things get murkier. While the Democratic Party would like to believe that the country has officially moved to the left, history and economics suggest the presidency is a coin flip. Yes, the nation has moved quickly on same-sex marriage. But even if human rights issues dominate social media profiles, economic conditions—like employment, wages and taxes—tend to win out in the voting booth. Here, the country is still split.

Finally, consider that the same party has only won three straight presidential elections once since World War II (George H.W. Bush after Ronald Reagan’s two terms). Hillary’s path to the White House is anything but guaranteed.

Biggest Hurdle: Voter Fatigue

Do Americans really want a third straight Democratic term?

Red Herring: Bernie Sanders

He’ll provide a left-minded foil for the debates, but no more.

Jeb Bush – The Moderate

Jeb Bush Presidential Candidate Profile | InsideGov

The slight favorite among GOP candidates, Jeb Bush’s last name is both the best and worst thing about his candidacy. On the positive side, he’ll win some support simply because people recognize his name. Political junkies might know their Walkers, Carsons and Fiorinas, but the average American would be lucky to remember anyone besides Clinton and Bush.

On the flip side, however, Jeb’s father and brother have middling reputations at best. In the public’s mind, George H.W. remains a footnote after the Reagan glory years, while George W. is widely considered a mediocre president, if not worse. Combine Americans’ general distaste for nepotism with their constant desire for change, and Jeb has a prickly path ahead.

The best news for Jeb is that he’s the most moderate of the leading candidates. If he can survive the primaries, he’ll be a compelling choice for the general election. Convincing GOP primary voters to skip more conservative candidates won’t be a picnic, but with growing support from the Republican establishment, Bush has a great shot to muscle his way past his right-leaning competition. After all, it’s the same formula John McCain and Mitt Romney successfully applied in their primaries.

2016 Ideology Spectrum | InsideGov

With an ideology score of 4.75 on a scale of -10 (very liberal) to 10 (very conservative), Jeb Bush is more moderate than Clinton (-6.5), Walker (6) or Rubio (6.75). Data from OnTheIssues.org

Biggest Hurdle: His Last Name

Will voters approve a third Bush for the White House?

Red Herring: Not Conservative Enough?

As long as his name doesn’t bog him down, Jeb Bush doesn’t need to overdo his conservative bonafides. He can win the GOP primaries as the levelheaded, logical choice.

Scott Walker: The Serious Conservative

Scott Walker Presidential Candidate Profile | InsideGov

The governor of Wisconsin has all the experience and accomplishments conservatives tend to love: over two decades of government service, executive leadership in a swing state and a record of fighting unions, both legislatively and judicially.

If the GOP decides to double down on conservatism, Walker is the safe, obvious pick. While Walker’s rivals were clowning around in Congress, the Governor was getting things done in Wisconsin. He’s the rare candidate who brings three parts steak for every one part sizzle.

That said, Walker’s lack of sizzle could ultimately be his downfall. After selecting two somewhat boring candidates—McCain and Romney—to face the charismatic Obama, GOP voters might worry that Walker’s muted, no-nonsense style could fizzle in the general election. Republicans want an executive with conservative credentials, but they’d rather be fired up than put to sleep.

Biggest Hurdle: Lack of charisma

Unless Walker can fire up supporters, his record may go unnoticed.

Red Herring: Far-right commentary

In wake of the same-sex marriage ruling, Walker was among the more critical voices, a position that grates with America’s popular opinion. But Walker’s levelheaded political instincts should prevent him from Trump levels of absurdity, even if the occasional comment raises eyebrows.

Marco Rubio: The Rising Star

Marco Rubio Presidential Candidate Profile | InsideGov

Another strong conservative, Marco Rubio is the flashiest, most inspirational figure in the 2016 field. The son of Cuban immigrants, Rubio has built a political career from scratch. Today, he is one of just three Latinos in the U.S. Senate.

Between his Cuban ancestry, service in a swing state, and loose ties with the Tea Party, Rubio can appeal to multiple key voting blocs: Hispanics, Floridians and Evangelicals. He’s also arguably the best of the five candidates on this list when it comes to stump speeches and storytelling.

Unfortunately, his wide appeal might be a little too wide. While Jeb can play to the establishment and Walker to the true conservatives, Rubio will need to walk a careful line between Tea Partiers, moderates and minorities. Should he take a hard line on immigration? Should he play up his swing state appeal? Each decision will require Rubio to alienate one or more of his potential bases, and you can bet his opponents will make him pick.

Biggest Hurdle: Defining his base

Rubio has broad appeal, but when pressed, which groups will he side with?

Red Herring: Inexperience

Having only served in the U.S. Senate for four years, some might label Rubio as inexperienced on the national level. Rubio will counter with his 11 years in the Florida House of Representatives, which should be more than enough to quell these concerns.

Rand Paul: The Sleeping Spoiler

Rand Paul Presidential Candidate Profile | InsideGov

Let’s be clear: Rand Paul is a long shot. His polling so far has been disappointing, and his campaign narrative has been muddy: is he a conservative or a libertarian?

Still, the reason you see Paul here—and not Christie, Cruz or Carson—is because Paul is unique. Unlike his GOP opponents, the Kentucky senator appeals to libertarians and young people, small but vocal groups who are increasingly shaping the country’s political discourse.

A simple combination of events could bring Paul from the fringe to the forefront: for instance, an economic regression paired with a spate of American casualties overseas. Voters could quickly embrace conservative economic principles while simultaneously favoring a hands-off international policy, a combination that would point directly—and almost exclusively—to Paul.

Unfortunately for Paul, he seems to be overly hedging his bets. He has moved to the right on many issues, including same-sex marriage, making him a less popular choice among young people. He’s likely trying to build up his conservative bonafides for the GOP primary, but he risks losing his unique, libertarian brand.

Biggest Hurdle: Uniting conservatives and libertarians

Rand would love to bring conservatives and libertarians together, but if he’s not careful, he’ll wind up alienating both.

Red Herring: Baggage from his dad

Rand Paul’s father, Ron Paul, is famous for his wild plans, like abolishing the Federal Reserve. In comparison, Rand has been far more modest with his proposals. In fact, Rand likely risks overcorrecting, rather than following his father too closely.

Explore Every Candidate on InsideGov