There's one Republican primary, but there are two contests that matter by far the most: Iowa and New Hampshire. Since the birth of the modern primary system in the 1970s, the path to the GOP nomination has always gone through one of them: No one has lost both and become the party's nominee.

And in that timespan, no Republican who wasn't already the president has managed to win both of them, either. That's because the evangelical, activist-dominated, low-turnout Iowa caucuses and the iconoclastic, independent-filled New Hampshire primary electorate play to very different candidates' strengths. And they play a key role in winnowing the field in the early stages — many candidates who fail to perform well in one or both of them quickly quit the race.

So if you're trying to make sense of this year's plethora of Republican candidates, it's helpful to separate out which ones are gunning for which early state. National Journal's very useful 2016 Presidential Candidate Travel Tracker tracks the number of trips each candidate has taken to each state. I've pulled out the trips to just Iowa and New Hampshire and put them in this chart to tally the trips each candidate has taken to Iowa and New Hampshire in 2015 so far. As you can see, there's quite a lot of variation:

We can can categorize the candidates in three main groups — the five who are clearly focusing on winning New Hampshire (on the left), the six who are gunning to win Iowa (on the right), and six more who are splitting their time between the two states (in the middle).

Candidates certainly don't have to choose either Iowa or New Hampshire, but it can be useful to set the expectations game. John McCain's fourth-place finish in Iowa in 2008 wasn't covered as a devastating blow to his campaign, because the press knew he wasn't seriously competing there. Then Mitt Romney's team downplayed the importance of the Iowa caucuses for most of 2011, but ended up nearly tying Rick Santorum in the state — a performance that could be spun as quite embarrassing for the establishment favorite, but was covered at the time as somewhat of a win for Romney.

The Iowa candidates: Scott Walker, Mike Huckabee, Rick Santorum, Bobby Jindal, Ted Cruz, Ben Carson

The candidates who are going to Iowa far more often than New Hampshire all have one obvious thing in common — they have strong ties with evangelical Christian conservatives. And this has made Iowa a crucially important state for all of them.

According to Pew's 2012 polling, 57 percent of the Iowa GOP caucus-goers identified as evangelical or born again, so ties to evangelicals can greatly help a candidate there. In comparison, a mere 22 percent of New Hampshire primary voters identified that way.

So ties to evangelicals can help a candidate in Iowa, but if he or she is viewed as predominantly a Christian right candidate, it might actually be a negative in New Hampshire. Take the two most recent GOP Iowa caucus winners, religious right candidates Mike Huckabee and Rick Santorum — neither managed to hit 12 percent in the Granite State primary days later. (Both are running again and, unsurprisingly, focusing on Iowa again.)

Because of the Huckabee and Santorum victories, Iowa Republicans have gotten a reputation for preferring religious right candidates with little mainstream support. But this hasn't always been the case for Iowa. Frontrunners Bob Dole and George W. Bush won the caucuses in 1996 and 2000, and eventually won the nomination.

Scott Walker hopes to follow that path. The Wisconsin governor lived in Iowa for part of his childhood and now governs one of the state's neighbors. Essentially he hopes to position himself as the most conservative candidate who can win, appealing to both evangelicals (his father is a preacher) and more mainstream party actors. Walker's not completely ignoring New Hampshire — he's visited it five times — but his best opportunity to break out early is clearly in Iowa.

In addition to Huckabee and Santorum, Walker's compatriots in an Iowa-first strategy are Ben Carson, Ted Cruz, and Bobby Jindal. All have spent years trying to appeal to evangelical Christians, and hope to emerge as surprise victors in the state. A poor performance in Iowa would be greatly damaging to any of their presidential hopes, since it doesn't look like their prospects in New Hampshire are particularly good, and many of them will likely drop out.

The New Hampshire candidates : Jeb Bush, John Kasich, Chris Christie, George Pataki, and Jim Gilmore

The batch of candidates traveling to New Hampshire far more than Iowa is the mirror image of the Iowa-first contenders. For the most part, they're more mainstream, less well-liked by evangelicals, and not as beloved to the conservative activists. Instead of throwing red meat at the base, they hope to appeal to mainstream Republicans and the independents who can vote in the New Hampshire primary.

Jeb Bush is the most prominent candidate counting on a win here — he's gone to New Hampshire 10 times, but Iowa only four. Despite being the fundraising frontrunner, Bush's prospects look very poor in the other three early states — Iowa and Nevada are caucuses, which tend to be dominated by more ideological voters, and South Carolina is, of course, quite a conservative state. So a strong performance in New Hampshire would be a crucial win for Bush early on.

Yet he faces competition from John Kasich, the prickly Ohio governor who's launched an underdog bid to take Bush on in the state. Kasich's team has been pretty straightforward that their whole strategy hinges on him doing well in New Hampshire, and indeed his operation is already advertising there, far earlier than most of his rivals. And the state's history could also inspire challengers to the establishment choice — Kasich's top adviser, John Weaver, worked for John McCain's 2000 presidential campaign when he unexpectedly beat George W. Bush in New Hampshire.

Then we have Chris Christie and George Pataki — one prominent Northeastern governor and one formerly prominent Northeastern former governor. Both are distrusted by social conservatives, but hope that New Hampshire independents will rally to their side.

The one somewhat strange exception here is Jim Gilmore, who served one term as governor of Virginia a while back and is quite conservative. This year, he's gone to New Hampshire eight times and Iowa just once, calling the former state "the most significant in the race." To me, though, his chances of victory here seem the same as they are in every other state — infinitesimal.

Candidates without a clear preference: Donald Trump, Rick Perry, Marco Rubio, Rand Paul, Carly Fiorina, Lindsey Graham

There are six candidates, though, who haven't shown a clear preference for either Iowa or New Hampshire, and have instead split their time among both states. Some seem to believe they could catch on in both, but others don't seem to be a particularly good fit for either.

These include the current poll leader in both Iowa and New Hampshire, Donald Trump. The billionaire's nativist rhetoric about Mexican immigrants could appeal to conservatives in both states. (Lest you think that New Hampshire always backs the establishment choice, recall that in 1996, Pat Buchanan, a conservative pundit who's harshly critical of unauthorized immigration, narrowly edged out frontrunner Bob Dole there.)

Meanwhile, Marco Rubio, who many Republicans think is the most charismatic candidate in the race, is distrusted by conservatives for being too pro-immigrant. He won his Senate seat in 2010 by winning over Tea Party activists and the conservative base, but he also hopes to compete with Jeb Bush for establishment support — and, seemingly, he hasn't chosen one of either Iowa or New Hampshire as his best opportunity to make a splash.

The other candidates competing heavily in both are Rand Paul (who hopes to make an impact in the Iowa caucuses with highly motivated libertarian voters, while also winning independents in New Hampshire), Carly Fiorina (who seems like a better fit for New Hampshire, but has visited Iowa a fair amount too), Lindsey Graham (whose heterodoxy hurts him in Iowa, but who doesn't seem like a good cultural fit for New Hampshire), and Rick Perry (who doesn't seem to be doing well anywhere).

Again, these candidates don't have to prioritize one state over the other — indeed, Jonathan Bernstein of Bloomberg View argues that no candidate should truly "skip" an early state. But there are so many contenders in the field that sooner or later they may have to make some tough calls about where they expend their resources and budget their own travel time. So we'll keep an eye on the data to see if candidates start revealing a preference for one or the other.