The most interesting Senate race in 2018 might have nothing to do with whether Republicans or Democrats control the chamber after this year's midterm elections.
The decision Tuesday by Sen. Orrin Hatch of Utah to retire teed up Mitt Romney -- the 2012 Republican presidential nominee turned leading intra-party critic of President Donald Trump -- to run for a job that would give him major influence over the remainder of Trump's presidency.
Right now, Democrats -- or independents who caucus with them -- are defending 25 seats this year
Republicans are defending just eight
But Hatch is a Republican. Romney is a Republican. And if Romney is stopped, it'll be because another Republican won the nomination in Utah. It'll be a fascinating state to watch -- but it won't tip the partisan balance of the Senate.
For that, look to two of Utah's neighbors: Nevada and Arizona.
If a blue wave is building ahead of this year's midterm elections, it will crash there first. The two states are the prime pickup opportunities for Democrats on 2018's map, and their recipe for winning the Senate includes victories in those two races plus holding all of their own seats.
Right now, Democrats -- or independents who caucus with them -- are defending 25 seats this year, while Republicans are defending just eight. The GOP has a 51-49 advantage at the moment.
Here's a look at the 10 Senate seats most likely to switch party hands in this year's midterms, as of January 2018:
Incumbent: Republican Sen. Dean Heller
There's no easy way out of the box Heller is in.
Danny Tarkanian, who's lost all five of his races in Nevada -- but won four of his five primaries -- is challenging Heller from the right, charging that he's insufficiently supportive of Trump.
Rep. Jacky Rosen, meanwhile, lies ahead in the general election, ready to seize on a favorable environment for Democrats in a state that Hillary Clinton won in 2016, and where Heller barely won -- and did it with less than 50% of the vote -- in 2012.
Heller's gymnastics on health care helped earn him the top spot in our initial rankings. He's still there as the calendar flips to 2018.
Open seat (Republican Sen. Jeff Flake is retiring)
Senate Majority Leader Mitch McConnell last month let slip what was already a poorly kept secret: Rep. Martha McSally is poised to enter the Republican primary here.
It'll become fight in the GOP civil war of former Trump adviser Steve Bannon vs. McConnell, with Bannon aligning conservative outsider forces behind state Sen. Kelli Ward -- a candidate many Republicans in Washington and Arizona see as a sure-fire general election loser.
If McSally prevails, as a former fighter pilot and strong fundraiser running in a red-leaning state, she'd have a real chance.
But Arizona is moving to the left -- it and Georgia are turning blue as rapidly as Ohio and Iowa are becoming red -- and Democrats landed their top target in Rep. Kyrsten Sinema. Strong minority turnout and an anti-Trump sentiment among college-educated suburban white voters make this a prime pickup opportunity.
One factor that could change that math: The health of Sen. John McCain, who has a brain tumor.
Incumbent: Democratic Sen. Claire McCaskill
This is the state where the Steve Bannon vs. Mitch McConnell civil war is on hold and things are largely going according to plan for Republicans.
McCaskill is one of five Democratic senators up for re-election in a state Trump won by double digits in 2016. Unlike the special election in Alabama and contests in several other states -- including Nevada and Arizona -- Bannon, Trump's former chief strategist, and McConnell, the Senate majority leader, are both backing Attorney General Josh Hawley here. McConnell pointed to Hawley at a news conference a month ago as the type of candidate he believes "can actually win."
This race will be expensive and brutal. Expect Democrats to cast Hawley, who just won his job a year ago, as a ladder-climber.
Incumbent: Democratic Sen. Joe Donnelly
Republicans point to two main reasons they can pick off this seat:
1. Donnelly is an accidental senator of sorts, winning only after tea partier Richard Mourdock ousted Sen. Richard Lugar in the primary before collapsing in the general election in 2012.
2. Republican Todd Young shellacked Evan Bayh by 12 points here just a year ago -- and Trump won Indiana even more handily.
But there are a few important differences. Most importantly: None of the three Republicans running this cycle -- Reps. Luke Messer and Todd Rokita and state Rep. Mike Braun -- is as talented, or baggage-free, as Young. (In particular, Messer faces residency questions, and people who work for Rokita have accused him of creating a toxic environment.)
Donnelly has kept a low profile and isn't particularly well-defined, but the "Mexico Joe" label that followed an Associated Press report that he profited from a family business's move to outsource jobs to Mexico still stings.
5. West Virginia
Incumbent: Democratic Sen. Joe Manchin
Manchin's early flirtations with Trump -- which included conversations about a Cabinet post -- may help insulate him in a state the President carried with 69% of the vote in 2016.
But Trump is showing signs he's willing to wield his popularity against the popular former governor, casting him as unserious about bipartisan cooperation.
"He talks. But he doesn't do anything. He doesn't do," Trump told The New York Times recently.
6. North Dakota
Incumbent: Democratic Sen. Heidi Heitkamp
Trump met Tuesday with Rep. Kevin Cramer, who it's safe to assume is now very likely to run against Heitkamp in this heavily Republican state. Getting him into the race would solve the GOP's biggest problem here -- figuring out who their candidate will be. Former state Sen. Tom Campbell is also running.
In a neutral political environment, the math suggests this seat would be an easy pickup for the GOP: Trump won North Dakota by an eye-popping 36 points in 2016, and Heitkamp won her seat by less than 1 point in 2012.
But this year's crop of Democratic incumbents features several candidates -- Heitkamp, Manchin and Montana's Jon Tester stand out -- with strong individual brands at home that have allowed them to defy their state's partisan makeup. And the nearly $4 million Heitkamp has in the bank goes a long way in North Dakota.
Incumbent: Democratic Sen. Sherrod Brown
Brown's populist brand of politics is in some ways -- trade, in particular -- similar to Trump's. That could be a good thing for Brown, since Trump won Ohio by 9 points last year.
Republican state Treasurer Josh Mandel lost to Brown in 2012, and losing candidates' repeat efforts seldom lead to better results.
Like Missouri, the question here is whether Ohio's rightward trend will prevail in 2018, or this race will instead swing with what appears to be a favorable national environment for Democrats. As of right now, the 2017 results in Virginia, New Jersey and Alabama suggest the latter. Brown will need that to be the case.
Incumbent: Democratic Sen. Bill Nelson
Wake me up when Republican Gov. Rick Scott makes up his mind.
If Scott's in, his proven ability to win close races in Florida and self-fund in an extraordinarily expensive state would send Florida up this list in a hurry.
Not only that, he could force Democrats to spend heavily on their effort to retain Nelson's seat -- which could limit the party's ability to go after long shots like Tennessee.
Scott lunched with Trump on New Year's Eve. But on Tuesday he punted when reporters in Jacksonville asked him about a Senate run, noting he has 390 days left in Tallahassee.
Incumbent: Democratic Sen. Jon Tester
Republicans missed on their top recruiting targets here. And now, state auditor Matt Rosendale and businessman Troy Downing are headed toward what could be an expensive and nasty primary.
That's all good news for Tester, whose populist, down-to-earth brand -- bolstered by the fact that he still runs a farm -- allowed him to win in 2006 and 2012 despite Montana's history of voting for Republican presidential nominees (Trump won by 21 points here).
Incumbent as of midday Tuesday: Democratic Sen. Tina Smith
Minnesota is ground zero in the 2018 midterms: The governor's office, as many as five House seats and not one but two Senate seats are up for grabs: Democratic Sen. Amy Klobuchar's and this one, vacated by Al Franken.
Smith, Minnesota's lieutenant governor, was appointed by Gov. Mark Dayton to replace Franken.
The Republican field is still taking shape. But an open seat in a state that's moving to the right over time looks, for now, like a slightly more appealing target for Republicans than neighboring Wisconsin, which just missed this list.
Elsewhere on the map:
In Wisconsin, Democratic Sen. Tammy Baldwin will wait and see who emerges from a packed Republican primary. One to watch: state Sen. Leah Vukmir, a talk radio darling in a state where conservative talk radio really matters.
Democrats got their dream recruit in former Gov. Phil Bredesen in Tennessee, where they hope to pick off retiring Sen. Bob Corker's seat. But is Bredesen still the force he was 15 years ago, or is he this cycle's Evan Bayh?
New Jersey Democratic Sen. Robert Mendendez's corruption trial ended in a mistrial, but the fact that he faced the charges at all makes his fate worth watching.
Republicans would love to put Democratic Sen. Bob Casey's seat in Pennsylvania in play, and they have a Trump ally in Rep. Lou Barletta to try it.
Democrats, meanwhile, hope Rep. Beto O'Rourke can catch fire in Texas against Sen. Ted Cruz if the national environment swings even more in the party's favor.