In the spring of 2010, Senate majority leader Harry Reid (D., Nev.) was widely considered the most vulnerable incumbent up for reelection. His campaign interceded in the Republican race for the nomination to take him on, attacking the candidate he considered most threatening. Sharron Angle, the candidate he feared least, won the nomination and then lost in November.

This year, Senator Claire McCaskill (D., Mo.) is the most vulnerable incumbent up for reelection, and she has learned from Reid’s example. She is trying to get the candidate she fears least to win the Republican nomination. But she is going about it in a slightly different way: “attacking” that candidate in terms likely to make Republican primary voters warm to him.

The polls in the Republican primary have been fluid, and it seems as though businessman John Brunner, Representative Todd Akin, and former state treasurer Sarah Steelman are all in serious contention one week before the vote. Conservatives are split. Steelman has Sarah Palin’s endorsement, for example, while the Chamber of Commerce is blasting her voting record on taxes, tort reform, and economic conservatism generally.

McCaskill, though, seems to have a clear favorite in the race: Todd Akin. Watch her ads against the Republican trio here. The ad “against” Akin calls him a “true conservative” and “a crusader against bigger government,” and refers to his “pro-family agenda.” The ad against Brunner, on the other hand, attacks him for not being a “reliable conservative.” The ad against Steelman is tough, too.

Harry Reid’s PAC is playing in the race, too, trashing Brunner’s business record.

All of which seems to suggest that Democrats want to run against Akin the most, and may want to run against Brunner the least. The Democrats may believe that Akin’s relative lack of interest in fundraising and his religious conservatism make him the weakest of the Republican candidates. Parties have, however, been known to get these sorts of calculations wrong.

It is hard to see how the Republicans can take the Senate without this seat. At the moment Rasmussen shows all three Republicans beating McCaskill, with Akin having the smallest margin.