Chief correspondent

If you think all politics are local, as former House Speaker Thomas P. “Tip” O’Neill (D) always said, think again. The same forces that have created the deep, partisan divide in presidential elections are now changing the nature — and outcomes — of House and Senate elections.

That’s the conclusion of a new study by Alan Abramowitz and Steven Webster of Emory University, presented this weekend at the Midwest Political Science Association. It is an extension of the work Abramowitz has done examining the impact of partisanship and polarization. The findings offer worrisome news about the implications for the Democratic Party’s hopes of taking back control of Congress.

The authors coin the term “negative partisanship” for a condition that has been increasingly evident in politics, which are the increasingly negative feelings those from one party have for those in the other party. People don’t like their own party any more or less than they used to, but they dislike the other party much more.

Racial, cultural and economic attitudes have contributed to the political separation of the population. Republicans are more conservative than they were in the past while Democrats are more liberal. The result, Abramowitz and Webster write, is that there has been “a very large increase in the ideological distance between supporters of the two parties” — more than a doubling in the past four decades.

The larger this gap has become, the more that partisans on each side see important differences between the major parties. And seemingly, the more they see those differences, the less they are likely to vote for a candidate from the other party — despite a rise in the percentage of people who call themselves independent or who register with no party affiliation.

Presidentially, Democrats enjoy an advantage in the electoral college arithmetic. Their electoral foundation, based on the past half-dozen elections, is bigger than that of the Republicans — although certainly not insurmountable. But the district-by-district vote for president produces a different result. Because of today’s polarized voting patterns, that has had a big effect on congressional elections.

In the past, House members could count on their own attributes and skills to prevail in a district whose partisan leanings went against them. Today, with a sharp increase in straight ticket voting, that becomes more and more difficult.

What seems to matter most today is the pattern of the presidential vote, district by district, rather than local factors, including the power of incumbency. House and Senate races have become nationalized.

Here is some of the data they offer to back up their conclusion. From the 1960s to the 1980s, Republicans won about 60 percent of House races in districts that voted more Republican in presidential elections than the GOP’s national average. Democrats won about 80 percent of contests in districts that were more Democratic than the national presidential average.

Starting in 1994 and accelerating in 2010, Republicans have done significantly better in those GOP-tilted districts. Before the 2010 election, the GOP held 165 of the 232 Republican-leaning districts. That year, they won 52 of the 67 GOP-leaning districts that had been held by Democrats. That gave them 94 percent of those seats, rather than 71 percent previously. The same patterns occurred in 2012 and 2014.

Democrats also have consolidated Democratic-leaning districts. But they remain at a big structural disadvantage. Abramowitz and Webster estimate that 240 of the 435 congressional districts now lean Republican, while only 195 lean Democratic. If each party continues to win 95 percent of the districts whose presidential votes go in their direction, Republicans will remain in the majority for some time.

Many people blame these structural conditions on partisan gerrymandering by Republican-controlled legislatures. That is only part of the reason. The other, which Democrats sometimes overlook, is the distribution of the population. Democrats are packed more closely in metropolitan areas, while Republicans are spread across urban, suburban, exurban and rural areas.

The population distribution isn’t new. What is new is the ability of Republicans to convert their presidential advantage in those districts into victories in House races.

Abramowitz and Webster say the changes are even more dramatic in Senate contests because the link between presidential and Senate voting was often weaker in the past. Today it is four times as strong as it was only a few decades ago.

“The electoral fortunes of Senate incumbents, like the electoral fortunes of House incumbents, now depend less on their personal popularity and more on the partisan makeup of their states than in the past,” the authors note.

Democratic-leaning states are more likely to elect Democratic Senate candidates than they once were, and Republican-leaning states are more likely to elect Republican Senate candidates. Last November, according to the authors, the “advantaged party” won 33 of 36 Senate races.

Last year’s Senate race in Arkansas offers an example of the connection between presidential and congressional voting — and the swiftness with which things have changed.

In 2008, then-senator Mark Pryor (D-Ark.) won reelection with 80 percent of the vote. Through much of the 2014 campaign, he remained competitive in the polls against his GOP challenger, then-representative Tom Cotton.

But Arkansas was undergoing a big shift toward Republicans in presidential elections. In 2000, then-Texas governor George W. Bush won Arkansas by 5 percentage points. In his presidential reelection campaign, he carried it by 10 percentage points. Four years later, Sen. John McCain (R-Ariz.) defeated then-senator Barack Obama by 20 points. In 2012, Obama lost to Republican Mitt Romney by 24 points.

What happened to Pryor? By Election Day, many prognosticators gave Cotton the advantage, but not by an overwhelming margin. A Real Clear Politics average of polls showed Cotton with a lead of about 7 points. When the votes were counted, Cotton defeated Pryor by 17 points — strikingly large for someone who had the experience, power of incumbency and local political pedigree of Pryor.

Democrats face a similar problem in the Senate as in the House. The distribution of the population favors Republicans in the ongoing battle for control. Thanks to the Founders, big states and small states each get two senators. Democratic voters are concentrated in metropolitan areas of the most populous states. More sparsely populated states tilt to the GOP.

With a third of the Senate up for reelection each election year, the GOP advantage varies cycle by cycle. In 2010, 59 percent of the states with Senate contests leaned Republican, according to the authors. In 2012, 48 percent leaned Republican. In 2014, 64 percent leaned to the GOP.

Over the past quarter-century, Democrats have been more successful at controlling the Senate than the House because they were winning in GOP-leaning states. If they cannot continue to do so, the authors write, “We may see a Republican advantage in Senate elections similar to the party’s recent advantage in House elections.”