Sen. Pat Roberts, R-Kan., landed an unenviable spot on a list of longtime U.S. senators vulnerable to a challenge from the right, in the wake of the Nebraska and Indiana GOP primaries. Salon.com's Steve Kornacki wrote of Roberts: "He's old (76) and has been on Capitol Hill for 32 years – the first 16 in the House and the last 16 in the Senate. He's also a quiet, behind-the-scenes player whose voting record is only now evolving to sync up with the GOP base's prevailing mood. It wouldn't be too hard for an opponent to portray Roberts as a tired insider with Potomac Fever. Plus, the Kansas Republican Party is unusually prone to civil war. Roberts could provide an inviting target for, say, Kris Kobach, the youthful Kansas secretary of state who has become the leading national voice of the anti-immigration right." The other vulnerable Republicans on the list: South Carolina's Lindsey Graham, Georgia's Saxby Chambliss, Tennessee's Lamar Alexander, Kentucky's Mitch McConnell, Mississippi's Thad Cochran and Maine's Susan Collins.