In the House of Representatives, not only do more than 90 percent of incumbents seeking reelection usually win, but most of them win with more than 60 percent of the vote

Discuss why the rate at which incumbents are reelected to the Senate is lower than at which House members are reelected.

What will be an ideal response?

An ideal response will:
1, Explain that there is greater competition in the Senate, in part because an entire state is almost always more diverse than a congressional district and thus provides a larger base for opposition to an incumbent.
2, Note that senators have less personal contact with their constituencies, which on average are about 10 times larger than those of members of the House of Representatives.
3, Note that senators also receive more coverage in the media than representatives do and are more likely to be held accountable on controversial issues.
4, Note that senators tend to draw more skilled and visible challengers, such as governors or members of the House, whom voters already know and who have substantial financial backing—a factor that lessens the advantages of incumbency.