Noah Kristula-Green documents it:
O’Donnell had a consistently negative effect on the close down-ticket races in Delaware.
Republican Party officials in the state who spoke to FrumForum on and off the record expressed great frustration with the damage she caused. O’Donnell did this in several different ways. She cost the GOP several candidates in the Delaware State House, giving the Democrats a super-majority. She hurt the campaign for the Republican nominee for State Treasurer. She boosted a Democratic party which has been growing stronger in the state, and solidified in the minds of many voters the view that the Republican party was an atavistic and unserious party, which the mainstream had rejected. In a year when Republicans had a wave to take advantage of and the opportunity to grow across the entire country, O’Donnell failed her party and brought it down.
Now imagine the down-ticket disaster Palin would be.