Republican hubris

Many Republicans got blindsided in the 2006 elections, and James David Dickson cites the example of an Iowa legislator in this article for The American Spectator:

It wasn't until Atlantic reporter Joshua Green called former state representative Danny Carroll that Carroll started to understand what hit him. He had been Speaker Pro Tempore of the Iowa State House, which was controlled by Republicans right up to the 2006 midterm elections. Then he went down to unexpected defeat, contributing to his party's loss of both houses of the state legislature.
Carroll's district is home to Grinnell College, so he had assumed that high turnout among the liberal student body, as well as low turnout for Republicans generally, had caused his defeat. It wasn't until Green called him to ask about the gay rights activism of Quark founder Tim Gill that Carroll realized his defeat had been part of a coordinated plan.
Gill had quietly spearheaded an effort to fund challenges to 70 anti-gay marriage state politicians in the 2006 election cycle. Fifty of them went down to defeat. Green convinced Carroll to look at the source of some of the larger donations to his Democratic opponent Eric Palmer, and was rewarded with good soundbyte: "Denver...Dallas...Los Angeles...Malibu...there's New York again...San Francisco! I can't -- I just cannot believe this!"

Read the whole thing. What happened to Carroll is typical of how incumbents get defeated. They start thinking their re-election is inevitable, they listen to aides and supporters who tell them that everything's wonderful, they fail to react in a timely manner to new challenges.

In most elections, the candidate who is elected with 55 percent of the vote is considered a big winner. But that 10-point margin of victory is more easily eroded than some incumbents think. Every re-election campaign is a new campaign, and if an incumbent doesn't go all-out -- if he doesn't fund-raise and organize as if his political career depended on it -- defeat is always a possibility.

Re-electing an incumbent is almost always easier than electing a challenger, but the advantages of incumbency are not so great that the incumbent can take re-election for granted.