THERE are many theories for why very conservative Republicans seem to be doing so well lately, taking their party’s Senate nominations in Florida, Kentucky and Utah, and beating Democrats head-to-head in Massachusetts, New Jersey and Virginia. Some attribute this to a generalized anti-incumbent mood. Others say it reflects the tendency of parties in power to falter in midterm elections. Recently it has been fashionable to ascribe right-wing success to the Tea Party movement.

But the most obvious explanation is the one that’s been conspicuously absent from the gusher of analysis. Republican success in 2010 can be boiled down to two words: Rush Limbaugh.

Mr. Limbaugh has played an important role in elections going back to 1994, when he commanded the air war in the Republican Congressional victory. This time, however, he is more than simply the mouthpiece of the party. He is the brains and the spirit behind its resurgence.

How did this happen? The Obama victory in 2008 left Republicans dazed, demoralized and leaderless. Less than six weeks after the inauguration, in a nationally televised keynote address to the annual Conservative Political Action Conference, Mr. Limbaugh stepped into the void with a raucous denunciation of the new president’s agenda and a strategic plan based on his belief that real conservatism wins every time. He reiterated his famous call for Mr. Obama to fail and urged the party faithful to ignore the siren song of bipartisanship and moderation and stay true to the principles of Ronald Reagan.