The center continues to collapse in Congress.

The 2014 election accelerated a trend of straight-ticket voting, the phenomenon of people voting for the same party for Congress as they did for president. With the ideological distance between Democrats and Republicans growing bigger than ever, the result is a Congress sharply divided along party lines, with a shrinking bloc of centrists more open to compromise.

"You have consistently liberal Democrats, consistently conservative Republicans and very few moderates," said Eric Heberlig, a political scientist at the University of North Carolina at Charlotte.

If Louisiana Democrat Mary Landrieu loses her runoff election next week, the Senate that convenes in January will have 84 senators of the same political party that carried their state in the most recent presidential election. That's the most in more than six decades, according to statistics compiled by Gary Jacobson, a political scientist at the University of California at San Diego. There were 61 such senators in 1999, after the second midterm election of President Bill Clinton's administration, and 43 in 1987.

There's also more partisan alignment in voting for the House of Representatives and for president.

The 2014 election produced just 31 districts that voted for one party's House nominee after backing the other party's presidential nominee in 2012, according to data compiled by Bloomberg Politics. That's just 7 percent of the 435 districts, and only a slight increase from the 26 ticket-splitting districts after the 2012 election. The number ticked up a bit because the Republican wave ushered in new members from districts that were mildly pro-Obama in 2012, when the president was more popular. But it's still historically low, far fewer than the 83 split districts produced by the 2008 election or the more than 100 split districts that were common from the 1950s to the 1990s, according to Vital Statistics on Congress

Just five of the 31 ticket-splitting districts were won by Democrats in the 2014 House elections, underscoring how the party's conservative and moderate membership has been decimated. Just three of the 34 House Democrats who voted against Obama's Affordable Care Act in 2010 will be serving in the chamber in 2015.

The number of ticket-splitting districts may dip again in 2016, when freshman Republicans who won districts that voted for Obama in 2012 could face tough re-election campaigns in districts that may revert back to their normal voting patterns in a presidential election year. In the Senate, seven Republicans are up for re-election in states that voted for Obama in 2012.

"Voters are more partisan than they used to be," Heberlig said. It wasn't uncommon for voters to "vote one way for president and another way for Congress and state and local offices. Now voters are more consistently just supporting Democrats or just supporting Republicans."

The differences between the parties are heightened in an era of win-at-all-costs campaigns and narrow majorities. "When the parties are so distant from one another, when they're engaged in this competitive game for control of the majority, you don't see that kind of bargaining and negotiation. And yet our system can't work without it," Thomas Mann, a senior fellow in governance studies at the Brookings Institution, said last week.

It wasn't always so.

The South, now a Republican stronghold, was a Democratic bastion for a century after the Civil War. Even by the 1960s and 1970s, when Republican presidential candidates carried Southern states, the region still split their tickets by staying loyal to Democratic candidates for Congress. In the 1980s and 1990s, the South became more comfortable voting Republican for Congress. Conservative and moderate Democrats retired or were beaten, succeeded by Republicans. 

"That's a big part of the story, and why the geographic polarization that we see right now is as rigid as it is. The South has sort of sorted out now," said Frances Lee, a political scientist at the University of Maryland at College Park.

As most of the South hardens into a one-party Republican fortress, the Northeast and Pacific Northwest have also sorted themselves out as Democratic bastions. There used to be an active liberal bloc of Republican officeholders in the Northeast, personified by people like Senators Jacob Javits of New York, Clifford Case of New Jersey and Edward Brooke of Massachusetts. That time has passed: Case in 1972 was the last Republican elected senator from New Jersey. Massachusetts just voted in an-all Democratic House delegation for the 10th election in a row.

California, once politically competitive, is now a Democratic stronghold; the party actually gained a House seat this year, bucking the Republican wave.

The polarization is likely to endure. "To move to a less polarized area," Heberlig said, "we're going to have to have some new issue that divides the current ideological coalitions of the party or some event that shifts people away from voting on these types of ideological issues that would remix or resort the current divisions."

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