So, what accounts for the difference?
Last year, the League of Women Voters bought television ads in Missouri faulting Democratic Sen. Claire McCaskill for voting to weaken the Environmental Protection Agency. Concerned the ads could hurt a vulnerable incumbent, Senate Majority Leader Harry Reid, D-Nev., summoned representatives from the group and environmental organizations to a May 2011 meeting to tell them that attacking McCaskill would hurt their agenda. The spots stopped. McCaskill today can tout a moderate voting record without flak from the left and hit the campaign's homestretch a favorite.
Compare Reid's heavy hand with Senate Republican leaders' floundering in the same state. In Missouri's three-way Senate primary, they saw business executive John Brunner as the best option and Rep. Todd Akin as a weaker general-election candidate. But the National Republican Senatorial Committee stayed out of the contest, part of a revised approach instituted after the committee drew complaints in 2010 for opposing tea party favorites in primaries.
But claims that campaign-committee performance can be chalked up solely to leaders' competence overlook emerging differences between the parties. Under President Obama, the GOP has become the more fractious party. The Republican insurgent-versus-establishment divide defies the stereotypical view of the GOP as more able to impose party discipline than the bigger-tent Democratic camp.
Republicans "in many ways are a little more diverse" than Democrats, McConnell said in an interview this summer with National Journal, noting that Democrats "have more ... ideological harmony."
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