Why incumbents win elections

Louis professor Gautam Gowrisankaran and two colleagues. We seek to explain the causes of the incumbency advantage, using data on the outcomes of Senate elections since The study considered three potential explanations for the incumbency advantage: tenure, selection and challenger quality.

Gowrisankaran began researching the issue after the election along with Matthew F. The study encompasses every Senate election from the start of the elected Senate in through The researchers found that the effects of tenure — the advantage of being in office itself — are negative or small. They also found that incumbents face weaker challengers than candidates running for open seats, those that are available when the sitting senator is not running for re-election.

Further, the researchers predict that if incumbents faced challengers as strong as candidates for open seats, the incumbency advantage would be cut in half, and senators would win reelection only 64 percent of the time. Two senators with identical tenure could have important differences in their electoral history. For instance, a one-term incumbent could have beaten a two-term incumbent, or he could have beaten a five-term incumbent.

The study is not a predictive tool for who will win elections, although the researchers do make predictions on the election. Interestingly, John Edwards of North Carolina, the Democratic vice-presidential nominee, would have had only a 67 percent chance of winning had he chosen to run again, less than any incumbent actually running for reelection. The formula assigns him this probability based on the fact that he is a one-term senator who beat a one-term incumbent, who had also beaten a one-term incumbent who won an open seat.

On the other hand, Patrick Leahy of Vermont has a 90 percent chance of winning because he is a five-term incumbent who first won an open seat. It's being billed as one of the most consequential midterm elections in decades , with a wave of Democrats hoping to capitalize on widespread voter dissatisfaction to oust their GOP opponents and win control of Congress. Thirty-five senators and all House members face the prospect of losing their jobs in November.

But political analysts tracking the races say that only a relatively small number of races — fewer than 50 House seats and just a handful of Senate races — are truly competitive. A look at past election cycles helps explain why.

Since , voters have sent their incumbent House representative back to Washington 93 percent of the time. Senators enjoy only slightly less job security — 82 percent.

Academics have speculated on the multiple reasons that congressional incumbents have enjoyed an advantage over the years. Incumbents have traditionally used their positions to win favor with voters by offering a variety of constituent services or by pointing to increased funding they've captured for the home state or district. More recently, some have argued that redistricting has created politically lopsided seats that strongly favor one party over another.

Moreover, as the cost of mounting a political campaign has risen, incumbency in Congress has created an important financial advantage in attracting the money needed to win. Based on the amount of campaign cash both sides have raised so far, Republicans are in a better position to defend their majority in the House than the Democrats are in taking it away.

If the Senate's two independents continue to caucus with them, Democrats only need to pick up two seats to win a majority. And five of the eight seats that are considered "toss-ups" by one or more of the political pundits rating the races are currently held by Democrats. That may be why Democrats in those races have been busy fundraising.