An incumbent usually wins an election because of the perks of his office, which include budget for a staff in Washington, D.C. and at home, and a travel allowance, which allows him to connect with constituents while in office. Incumbents also tend to raise more money than their challengers.
In the November 2004 election, incumbents in the House of Representatives raised nearly $457 million, while challengers raised a little over $112 million. In the same year in the Senate race, incumbents raised just under $224 million versus just below $80 million by challengers. The average House incumbent outspent his challenger by $700,000 in 2004, and the average Senate incumbent outspent his challenger by $4 million.
In addition to money, an incumbent spends his time in office connecting with constituents, attending special events, and appearing on television or radio talk shows, essentially the same activities a candidate does during a campaign. The incumbent receives a full-time wage while doing so, versus the challenger who has to find alternative ways to pay his bills while campaigning. These activities make an incumbent widely known in his district or state.
An incumbent has also already run at least one successful campaign, which means he has valuable experience in creating an efficient campaign organization. This also means an incumbent generally has an effective volunteer organization in place when election time comes.