Incumbents typically have an advantage over challengers in Congressional elections in the United States. There are several reasons for this.
First, an incumbent has the power to deliver things to his constituents. For instance, he can persuade a company to open a factory in his district. He might use his influence to award his supporters with an infrastructure project. A challenger can only make promises.
A second advantage that incumbents have is in campaign financing. Running for office is an extremely expensive undertaking. An incumbent has a far easier time raising the funds needed for lengthy and costly campaigns. Sponsors and donors are typically more generous with incumbents than their challengers.
A third reason why incumbency is an advantage is name recognition. Name recognition is not only an asset for fundraising; it is also an advantage with voters. On election day, voters are more likely to elect a candidate whose name they recognize.
Another reason why an incumbent has an edge is gerrymandering. Gerrymandering is drawing Congressional districts in a way to benefit a political party. In a gerrymandered district, the politicians choose their voters and maximize their chances of winning reelection.
Incumbents have a large advantage. They usually win reelection and remain in office for many years. Because of this, many American citizens believe that term limits are needed.