As the midterm elections approach, one of the more interesting storylines emerging is in states that have both a high-profile gubernatorial race and a high-profile U.S. Senate race.

It got us thinking about how these contests might impact each other, especially in what's shaping up to be good year for Democrats. Looking back, we found a few patterns from previous midterms that gives us a clue about what to expect this year in these dual contest states.

There are four states hosting both competitive gubernatorial and U.S. senate races in 2018: Florida, Ohio, Minnesota and Nevada. And two other states have one competitive seat and one that is on the cusp of competitiveness -- Arizona and Wisconsin. (We determined whether a race was competitive using our most recent gubernatorial handicapping and the Senate handicapping by the nonpartisan Cook Political Report.)

The number of states with dual competitive contests is in line with recent midterm election cycles. In 2014, seven states ended up with competitive gubernatorial and Senate races: Alaska, Arkansas, Colorado, Kansas, Georgia, Michigan and New Hampshire. And in 2010, the previous midterm election, there were also seven states: California, Connecticut, Florida, Illinois, New Hampshire, Ohio and Wisconsin.

Looking at the results, these two election cycles saw the same party winning both races in a modest majority of cases -- four in 2014 and five in 2010. In the remainder, voters ended up splitting their tickets.

So what else can history tell us about paired competitive gubernatorial and Senate races?


The Direction of the Wave Matters

Both the 2010 and 2014 midterms were GOP wave elections. This acted as a kind of insurance in 2014 in three states with solid partisan leanings that voted for that party in both the gubernatorial and Senate contests.

One of those states was Arkansas, where the GOP was completing its transition to be the state's dominant party. The Democratic-held gubernatorial seat was open, while Democratic U.S. Sen. Mark Pryor faced a tough reelection bid against Republican Tom Cotton.

National political issues -- namely negative attitudes against then-President Barack Obama -- outweighed local ones, says Hal Bass, an Ouachita Baptist University political scientist. Both Democratic nominees, he says, "ran traditional, Arkansas-centric campaigns that sought to appeal to voters on a personal basis."

By contrast, both Republicans "ran more party- and ideology-centered campaigns that featured negative partisanship and demonizing their opponents as national, liberal Democrats," he says. "This latter strategy proved quite successful." The Republicans easily won both the gubernatorial and Senate races.

The past few election cycles in Arkansas demonstrate that "our ticket-splitting propensities have declined dramatically, in keeping with the rest of the country," adds Bass.

Georgia was another state whose old Democratic leanings were steamrolled by national Republican momentum. In 2014, the Democrats ran candidates with storied Georgia political names: Michelle Nunn, who sought to follow in her father Sam Nunn's footsteps to the U.S. Senate, and Jason Carter, the grandson of President Jimmy Carter, who ran for governor.

In the end, neither were a match for their Republican opponents. Incumbent Gov. Nathan Deal beat Carter to win a second term and Republican David Perdue defeated Nunn.

In the 2010 midterms, the Tea Party movement helped Republicans win both the gubernatorial and Senate races in Florida and Wisconsin. The races in Wisconsin, for instance, "were fueled by anger at the Obama administration," says University of Wisconsin political scientist Barry Burden. Republican gubernatorial candidate Scott Walker emphasized his opposition to Washington, and Ron Johnson, the Republican Senate challenger to Democratic incumbent Russ Feingold, "appeared on the scene out of nowhere, motivated almost entirely by his opposition to the Affordable Care Act."

But state partisan leaning can trump a national wave.

Even in the strongly Republican election of 2010, strongly Democratic states saw sweeps of both contests. The Democrats won simultaneous gubernatorial and senatorial contests in both California and Connecticut in 2010.


National Money Can Swamp Local Issues

Even contests that initially seem to be driven by local factors can be nationalized once big national money enters the picture.

The clearest example of this pattern was in Kansas in 2014, where incumbent Gov. Sam Brownback and long-serving Sen. Pat Roberts -- both Republicans in a historically Republican state -- faced serious challenges. Brownback was seeing blowback from a tax cut that produced big budget shortfalls; Roberts, after underperforming against a flawed Republican primary challenger, was running against a well-funded independent challenger, Greg Orman, who was so formidable that the Democratic nominee decided to exit the race.

However, national Republicans rushed to invest in Kansas because Roberts' suddenly competitive race could have cost Republicans their shot at winning control of the Senate. "The Roberts race became nationalized overnight, in early August," says University of Kansas political scientist Burdett Loomis.

The newly nationalized Senate race, with a focus on Obama and the Affordable Care Act, "reinforced lots of Republican sentiment in an already Republican state," Loomis says. "Brownback benefited from the nationalization of the race -- a trend that would have been far less prevalent if Roberts had not been in trouble."

As a result, Brownback ended up joining Roberts in the winner's circle.


More Competitive, Swingier States Likelier to Split Tickets

Three states saw split results in 2014. One was purplish Colorado, where Democratic Gov. John Hickenlooper was reelected and where Democratic Sen. Mark Udall lost his reelection bid to GOP challenger Cory Gardner.

In Colorado, "the gubernatorial and Senate races were not a bundled group of contests collectively driven by a wave," says Colorado State University political scientist John Straayer. "The outcomes simply resembled what Colorado is -- evenly divided between red and blue statewide."

Meanwhile, in similarly purplish Michigan, the Tea Party push didn't materialize to the same degree as it did in other states that were more solidly Republican, says Bill Ballenger, who publishes The Ballenger Report, a Michigan political tipsheet. Republican Gov. Rick Snyder narrowly won reelection, but Democrat Gary Peters easily defeated Republican Terri Lynn Land for an open Senate seat by double digits.

"Michigan's 2014 gubernatorial and senatorial general elections were totally separate animals," says Jake Davison, the editor of the political newsletter Inside Michigan Politics. "The gubernatorial race was a contest between two strong candidates, while the Senate race was a total mismatch. Despite 2014 being a banner GOP year, Land not only failed to land a punch, she never seemed to even get her gloves on."

Alaska was another state that saw a split ticket in 2014. While the state generally leans Republican, an odd lineup in the gubernatorial race kept the GOP from sweeping both contests. In the general election, Republican Gov. Sean Parnell ended up facing Bill Walker, an independent running on a ticket with Democrat Byron Mallot.

Meanwhile, midterm sentiment against Obama hurt incumbent Democratic Sen. Mark Begich, who was seeking another term, "but it hurt Walker less" because he wasn't a Democrat, says Gerald McBeath, a University of Alaska-Fairbanks political scientist.


So What Do These Patterns Tell Us About the 2018 Contests?

If, following historical patterns, the Democrats end up with an edge this fall, they would be on track to take a majority of these contests.

In Minnesota, for instance, the gubernatorial seat is being vacated by Democrat Mark Dayton, and Democrat Sen. Tina Smith, who was appointed to her seat, is seeking to keep it. There, a Democratic wave would certainly benefit Democratic candidates, but Republicans are also feeling more optimistic than they have been in recent election cycles. The surprisingly strong showing in the state by Donald Trump in 2016, along with a more general shift of rural, blue-collar, but historically Democratic regions toward the GOP, has emboldened the party.

There's one hitch: Our list includes some notable swing states -- Florida, Nevada and Wisconsin -- and is devoid of strongly blue or strongly red states. So it would not be surprising to see examples of states splitting their tickets between Senate and gubernatorial races.

Take Nevada, where Republican Gov. Brian Sandoval is term-limited, and Republican Sen. Dean Heller is seeking reelection. Both the gubernatorial and Senate contests "should be close," says Jon Ralston, editor of the Nevada Independent. "I think the Democrats are more confident about the Senate than the governor's race, but both races could go either way."

There is one factor unique to 2018 that should play a role almost everywhere. "I expect this year that both the gubernatorial and Senate campaigns in Ohio will be affected by the enthusiasm created in support of or opposition to President Trump," says Mark Weaver, a Republican strategist in Ohio.