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STORY HIGHLIGHTS

Washington (CNN) -- Establishment Republicans, backed by business-friendly outside groups, launched a counter-offensive this year against conservative Senate challengers after two election cycles of hard-right candidates winning GOP primaries but losing in November.

Republicans need to flip six seats this year to win back the majority and don't want the same scenario to play out again. The tea party's scorecard this year is far different than in 2010 and 2012, when it knocked off several establishment-backed candidates.

Short-lived tea party victories

It's too early to say if the party's over, but as Stuart Rothenberg of the non-partisan Rothenberg Political Report wrote earlier this month, "it's already clear that the pragmatist conservatives have stopped the anti-establishment's electoral momentum."

Midterm users guide: 15 things to know

March 4

Firebrand Republican Rep. Steve Stockman launched a last-minute bid against Texas Sen. John Cornyn, but he never seriously challenged the incumbent. Cornyn won by more than 40 points.

May 6

North Carolina state House Speaker Thom Tillis won big against his top two conservative opponents, winning enough of the vote in the primary to avoid a costly runoff that would have delayed his general election face-off against Democratic Sen. Kay Hagan, one of the nation's most vulnerable incumbents.

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Tea party scorecard