Visualizing the GOP's 2016 Bracket

And why Scott Walker has a long, treacherous road ahead.

The Republicans running for president in 2016 will all compete in the same primary, but they won't all be chasing the same voters, especially at the start. Instead, the candidates start out fighting to emerge as the front-runner among a smaller subgroup — in some ways like college basketball teams fighting their way through one side of a tournament bracket before the finals.

There are tons of constituencies within the Republican Party, but most broadly, the GOP breaks into two sides: A more "establishment"-oriented one and a more "grassroots"-oriented side. Now, new polling data from Iowa reveal just how much certain candidates find themselves going head-to-head for the same groups of voters.

On one side, Wisconsin Gov. Scott Walker — currently Iowa Republicans' top choice for the 2016 caucuses — is trying to wrest away voters who also take a liking to candidates such as Sens. Rand Paul and Marco Rubio, former Gov. Rick Perry, and retired neurosurgeon Ben Carson. On the other side, a largely different group of voters has gravitated toward Jeb Bush and Chris Christie.

Below, the data shows just how much some candidates' support overlaps — and why Walker's early lead is so tenuous. The governor currently holds a broad piece of turf in the all-important opening caucus state, but he's going to have to defend it from a horde of hungry competitors.

Quinnipiac University's most recent survey of Iowa Republicans asked not only who 2016 caucus-goers would vote for, but whether they liked or disliked (or had even heard of) a dozen different GOP candidates for president. Quinnipiac provided National Journal with data showing how voters who look favorably on one candidate view the rest of the field. For example, the voters who like Texas tea party Sen. Ted Cruz are very cool toward Jeb Bush: Just 31 percent of Cruz fans also like the former Florida governor.

By contrast, 68 percent of the (relatively few) Iowa Republicans who view New Jersey Gov. Chris Christie favorably also like Bush. The two are appealing to the same slice of potential voters, while Cruz is lined up on the other side of the bracket alongside Paul, Rubio, Perry, Walker, and Carson, who share as many as 80 percent of the same supporters.

So, while Walker led the field in Quinnipiac's poll with 25 percent support (Paul, Carson, Bush, and former Arkansas Gov. Mike Huckabee were bunched behind Walker at 10 to 13 percent), it's easy to see why staying atop the group could be problematic for the Wisconsin governor, despite his positive attributes.

Around two-thirds of the Iowa Republicans who view Walker favorably also like Cruz, Huckabee, Rubio, and Perry. When any of them make a big splash — as Walker did in January with a well-received speech in Iowa — the voters who might gravitate toward them will largely be the voters Walker is trying to capture and hold, too.

This happened over and over in 2012, when some observers said a boomerang effect was driving the GOP primary. A large slice of the primary electorate was open to supporting a number of different candidates, and several — from Michele Bachmann to Herman Cain to Rick Santorum to Newt Gingrich — had moments where that openness translated into primary support against Mitt Romney. That moment would fade and the candidate would slide back into the pack, but another boasting a similar group of supporters always popped up. (Some Romney supporters had a different name for it: the "whack-a-mole phenomenon.")

On the other hand, there are opportunities for Walker or others to further expand their hold on that side of the bracket. A healthy number of caucus-goers say now that they would support Carson, who has never run for elected office, but if he fades, 82 percent of the Republicans who view him favorably also like Walker, per the poll. Around 70 percent like Huckabee, Rubio, and others, too.

But that fragmentation makes that side of the bracket dangerous; the highs are high, but with support potentially splitting so many ways, the lows are very low, too. On the other side of the bracket, Bush and Christie are clearly competing over similar voters, and there may be fewer for them overall in Iowa. (Bush was viewed favorably by 41 percent of Republicans and Christie by 30 percent, the two lowest scores in the field except for little-known Ohio Gov. John Kasich.) But they will also split those voters fewer ways.

Rubio dips into their supporters a bit more than other candidates, while those who view Paul favorably don't overlap with any one candidate to the same degree as Walker or Cruz or others. Ultimately, these overlapping, multi-way Venn diagrams will explain more about which Republican wins the Iowa caucuses than who's ahead in the horse-race polls right now.