These lawmakers might want to consider crossing the aisle in the 115th Congress

Some Democratic senators who face reelection next year are gearing up for a tough sell: that they still represent their voters’ interests, even if they’ve stuck with a party many of those voters have abandoned.

The following charts explain which lawmakers on both sides of the aisle are the most mismatched with their constituents, as judged by their preference in recent presidential elections. These will be some of the most interesting races to watch in 2018.

We’ll start with this map of states colored based on an analysis of 2012 and 2016 presidential election results called the Partisan Voter Index (PVI).

Next, let’s line those states up from most Democratic to most Republican. They range from solidly Democratic Hawaii on the left, all the way to safely Republican Wyoming on the right. In recent decades, many have shifted further into one camp or the other.

Now let’s look at the political affiliation of each state’s two senators — Democratic, independent and Republican.

Next, we’ll measure senators’ ideologies, from liberal to conservative, using a measure called DW-Nominate.

Of course, most Republicans fall in the upper right-hand corner of the chart, with a conservative voting record and a state that votes more Republican for president. The opposite is true for Democrats, with a majority falling in the lower left-hand, liberal-Democratic corner.

But there is a small group of senators whose states recently voted against their party for president. That puts them in an awkward spot.

Democratic Sen. Heidi Heitkamp, for example, would be asking conservative voters from North Dakota (R+17) to support a reelection bid next year. Those same voters gave Donald Trump a 36-point win over Hillary Clinton in 2016.

At times, senators like Heitkamp will need to break ranks to show that they represent their voters’ interests, rather than their party. All three Democrats who voted in favor of Neil M. Gorsuch’s Supreme Court nomination fall into this category.

Sen. Dean Heller (Nev., D+1), one of just two vulnerable Republicans running next year, has tried to please both poles in his state, defending Planned Parenthood on Monday. Sen. Susan Collins (Maine, D+3) — not up in 2018 — voted “no” on some Cabinet picks.

Democrats have a lot of work to do, with many more seats up for reelection in 2018. What’s worse for them: Many are in states that recently supported Mitt Romney, Donald Trump or both for president.

Take a look at the House, where the pattern is similar. Most of the seats are expected to be fairly safe, though Democrats have been watching recent special elections to see whether they want to compete in districts previously thought to be safely Republican.

See how the Democrats form more of a line, and Republicans are a taller, round cluster? This shows Democrats tend to vote similarly on legislation, and Republicans have a wider ideological spread in their voting, challenging party leadership.

Here are the members of the House Freedom Caucus, some of the most conservative Republicans who played an important role in derailing health-care overhaul efforts last month.

And here are the initial lists of House districts the parties hope to flip in 2018. Unsurprisingly, each is targeting the swing districts that sit around the middle of the presidential voting spectrum.

Unlike their colleagues in safe districts, the shrinking group of lawmakers who represent areas that are evenly split or are firmly against their ideological lean must appeal beyond their base.

It’s unclear if the tough odds for Democrats will result in a poor showing, as Republicans face their own set of challenges. Chief among these are the current lack of Senate challengers and Trump’s low approval rating, which could jeopardize the House majority.

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