Attack ads are typically bitter, viciously calculated to destroy an opponent. Grave narrators and red chyrons, embellished with organ/piano/violin, are supposed to make viewers want to lock their doors.

Not in Missouri! One of Democratic Sen. Claire McCaskill's most recent attack ads, "hitting" Republican Rep. Todd Akin, practically sings his praises, as the St. Louis Post-Dispatch's Kevin McDermott points out.

The ad calls Akin a "true conservative" - "the most conservative congressman in Missouri," no less - a "crusader against bigger government" with a "pro-family agenda." It notes his biting criticism of President Obama ("Akin alone says President Obama 'is a complete menace to our civilization'"), who registered a 51 percent disapproval rating in Missouri in one recent poll.

Why would McCaskill take it so easy on Akin in an ostensibly negative ad? Well, politics.

Akin is running in a three-way Republican primary for the chance to challenge McCaskill in November, and of all the candidates vying to oppose her, Akin is thought to pose the weakest challenge. Missouri Republicans will hold their primary on Tuesday, and Akin is running third behind businessman John Brunner and former state treasurer Sarah Steelman, according to a survey conducted by Mason-Dixon on behalf of the Post-Dispatch. He also underperforms both those candidates against McCaskill in putative general-election matchups, leading her by five percentage points, compared to Brunner's 11 and Steelman's eight.

If Akin wins on Tuesday, McCaskill's odds in November should improve. And in a race against the tea partier Steelman and the businessman Brunner, Akin probably won't complain about being called "too conservative" on Missouri airwaves.