The Democratic Party has, for the moment, become a regional party.
Only 449 counties across the nation are reliably Democratic. Five times that many -- 2,226 -- are reliably Republican. The Democratic counties are generally found along the coasts and in large urban areas.
Nowhere is the party's abandonment of rural America more apparent than in West Virginia. For most of the 20th century, this was a solidly Democratic state. In the first presidential election of the 21st century, however, George W. Bush carried the state and it's voted Republican at the presidential level ever since. Last year, Donald Trump won the state with a crushing 42-percentage point victory over Hillary Clinton.
Still, at the state level, West Virginia remained a Democratic stronghold until 2014. In that year, the Mountain State gave Republicans control of the state legislature and elected a Republican U.S. Senator for the first time in more than half a century. Last week, Democratic Governor Jim Justice switched parties to join the GOP.
These trends help explain why the midterm election outlook is fairly bleak for Democrats. U.S. Senator Joe Manchin is up for re-election as a Democrat. There's no doubt voters in the state like Manchin, but there's also no doubt they don't like the crowd he hangs out with in Washington. As a result, Manchin is currently considered only a very slight favorite for reelection. Still, it's far from certain.
Beyond West Virginia, the regionalization of the Democratic Party means that eight Democratic Senators are vulnerable because they are running for reelection in unfriendly territory. Three probably have a 50/50 chance to be reelected: Joe Donnelly in Indiana; Claire McCaskill in Missouri; and Heidi Heitkamp in North Dakota. Manchin and four others are just narrow favorites: Jon Tester in Montana, Bill Nelson in Florida, Tammy Baldwin in Wisconsin; and, Sherrod Brown in Ohio.
On the other side of the coin, just two Republican Senators are considered potentially vulnerable: Dean Heller in Nevada and Jeff Flake in Arizona. Both have angered the Trump wing of the GOP and will face primary challenges before getting the chance to compete in a general election.
Put it all together and the regionalization of the Democratic Party limits the party's upside potential in 2018.
On a good night for the Democrats, they would pick up the GOP seats in Arizona and Nevada while not losing any of their own seats. But, even in that scenario, the result would be a 50-50 split in the Senate. With Mike Pence casting the tie-breaking vote as Vice President, Republicans would still be charge.
In contrast, on a good night for Republicans, they might hold on to their two seats and pick up the three toss-ups in Indiana, Missouri, and North Dakota. That would give them a 55-45 majority in the Senate and a greater ability to advance their agenda. It's even possible to envision the GOP having a better night if they can pick up some of the seats where Democratic incumbents are only slight favorites.
It's impossible, of course, to know which way the political winds will be blowing on Election Day in 2018. However, it seems clear that the shrinking of the Democratic Party to a regional base will keep Republicans in control of the U.S. Senate.