Even the weakest GOP senate primary candidate bests the incumbent by 5 points.

National Journal:

Sen. Claire McCaskill (D-Mo.) trails all three of her potential Republican rivals, according to a new survey conducted for two Missouri news outlets, cementing her status as this cycle's most vulnerable incumbent.

Businessman John Brunner (R) holds the largest advantage over McCaskill, leading by a 52 perent to 41 percent margin. Former state Treasurer Sarah Steelman (R) leads by a 49 percent to 41 percent margin. And Rep. Todd Akin (R) leads by a 49 percent to 44 percent margin.

McCaskill's big problems come among independent voters. She scores just 40 percent among independents against Akin, and she's mired in the mid-30s when she's matched up with Brunner and Steelman. And while McCaskill has said she would like President Obama to campaign with her, Obama wouldn't be much of an advantage -- the poll shows Obama trails Mitt Romney by a 51 percent to 42 percent margin.

The poll shows Brunner leading the Republican primary with 33 percent, compared with Steelman at 27 percent, Akin at 17 percent and five other candidates lagging far behind. Though this is his first run for office against two long-time elected officials, Brunner has bolstered his name recognition by spending millions on advertising introducing himself to Missouri voters.

Mason-Dixon Polling & Research polled 625 likely Missouri voters between July 23 and 25 for the St. Louis Post-Dispatch and News 4. The poll has a 4-point margin of error, meaning the Brunner's and Steelman's leads over McCaskill are statistically significant. The poll surveyed oversamples of 400 Republican and Democratic primary voters, each of which carry a 5 percent margin of error.

McCaskill's biggest asset -- her personal favorability -- isn't as big as she may have hoped. Just 37 percent of Missouri voters have a favorable impression of the first-term Democrat, while 44 percent see her unfavorably.

All three Republicans have lower favorable ratings as they build their name recognition, giving McCaskill the opportunity to boost their negatives. And she's trying -- McCaskill has debuted negative ads against all three candidates (Though her ad against Akin looks more like a positive spot aimed at boosting him in the Republican primary).

McCaskill may be the most vulnerable incumbent, but there are several open races with retiring Democratic senators that appear likely to fall to Republicans.

But as far as a takeover  of the senate by the GOP, that prospect is still iffy. Intrade gives the GOP a bare 51% chance to win the senate. Many Democratic incumbents have been spending heavily and appear to be shoring up their position. They will have more money than their challengers and it will be an uphill battle for many GOP candidates to defeat their Democratic rival.