A National Journal analysis of every Senate race since 2004 for which exit-poll data exist found it is now common in competitive Senate races for at least 80 percent of voters who approve of a president to support his party’s nominee and for at least 80 percent of those who disapprove of a president to vote for the other party’s candidate. Here's a look at some senators who bucked the trend and some who could do the same in 2012.
Sen. Olympia Snowe, R-Maine, won reelection in 2006 despite a Democratic takeover of the House and the rising unpopularity of President Bush.(Richard A. Bloom)
Sen. Scott Brown, R-Mass., who in 2010 won the seat Edward Kennedy held for decades, faces a strong competitor in Elizabeth Warren in the fall.(AP Photo/Charles Dharapak)
Incumbent Sen. Claire McCaskill, D-Mo., faces tough competition in November in her conservative state.(AP PHOTO/PABLO MARTINEZ MONSIVAIS)
Sen. Joe Manchin, D-W.Va., won his seat despite tea party fervor in his state and criticism of Obama's and congressional Democrats' agenda in 2010.(AP Photo/Jeff Gentner)
Former Sen. Byron Dorgan, D-N.D., won reelection in 2004 in a deeply red state by a wide margin, 68 percent to 32 percent, against Republican Mike Liffrig.(Liz Lynch)