What's different this year is that Republican leaders and their allies are hitting back—hard.
McConnell and his supporters have used their massive financial advantage to pound challenger Matt Bevin in advance of Tuesday's primary election. In Georgia's primary, also Tuesday, the front-runner is business executive David Perdue—not three more conservative candidates, including one endorsed by 2008 Republican vice presidential nominee Sarah Palin. In North Carolina's Senate primary earlier this month, the winner to take on Democratic incumbent Sen. Kay Hagan was establishment favorite Thom Tillis, the state House speaker, not Kentucky Sen. Rand Paul's chosen candidate.
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Establishment Republicans haven't cleared all their hurdles yet. In two weeks, veteran Republican Sen. Thad Cochran of Mississippi faces tea party upstart Chris McDaniel in a race that has become exceptionally nasty. If McDaniel pulls off an upset, that might even give Democrats a chance to win a Senate seat in this most conservative of states.
But Cochran is favored—just as establishment Republicans are favored to retain the upper hand throughout the primary season.
That won't guarantee that Republicans will pick up the six seats they need to recapture control of the Senate. Nor will the determination of Republican House and Senate leaders to declare a cease-fire in their budget and debt ceiling wars with the White House and focus on Obamacare, Benghazi and the IRS.
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But all those are signs that, if they don't win the Senate, the credit will go to Democratic candidates—not self-inflicted GOP wounds.
—By CNBC's John Harwood.