So the question isn’t whether Mr. Mourdock can win in November. It’s whether he can take Mr. Lugar out in next month’s primary. The momentum building behind his bid suggests he can, but the Lugar campaign is counting on a late TV-ad endorsement by Gov. Daniels to save the day.

The governor, who is popular with tea partiers, establishment Republicans and many independents, worked in Mr. Lugar’s Senate and mayoral offices in the 1970s and ’80s. He says he respects Mr. Lugar’s 40 years of public service and admires his willingness to work with the opposing party.

Mr. Mourdock isn’t endearing himself to the party’s elders by picking a fight with Grandpa. But Republicans need fighters in the Senate, not diplomats. President Obama and Senate Majority Leader Harry Reid have shown no intention of negotiating in good faith with Republicans. So accomplishing tax and entitlement reform will require the GOP to win the war of public opinion. Indiana’s gutsy and articulate tea partier is just the kind of guy Republicans need on the frontlines.