In Kentucky, polls show Senate Republican leader Mitch McConnell way ahead of tea party challenger Matt Bevin. For the GOP, the danger is not that Mr. Bevin would edge past Senator McConnell and lose to Alison Lundergan Grimes, a Democrat. Instead, Bevin’s attacks are undercutting McConnell among general-election voters. Bevin is branding him as something both parties despise: a Washington insider.