Operation Chaos.

Before the Ohio and the Texas primaries Rush Limbaugh encouraged his listeners to cross the aisle and vote for Hillary Clinton to keep the her in the race as long as possible. He did this for two reasons; he believed that his party had nominated John McCain because they wanted to run him against Hillary. If Democrats nominated Obama, it would sort of ruin the strategy.

Secondly, he thinks that John McCain and most of his party are a bunch of jackasses who won't stand up to Obama or Hillary, so it's best to have them bloody each other as much as possible, since there won't be much bloodying in the campaign proper.

Not sure about that last one; recent history shows that Republicans are pretty damn good about going on the offensive.

At any rate, the Carpetbagger Report has information on how well this tactic succeeded. Apparently it was moderately successful.

Edited to Add: I thought I should provide a bit of Limbaugh's gloating over Operation Chaos.

You know, I was back in Snerdley's office. We were just chilling out back there. He said, "Do you realize what you just heard?" I said, "What do you mean?" "You have forced Barack Obama to counter you in Pennsylvania. He has had to start spending his own money, his campaign resources, on a campaign to get Republicans to register in the Democrat primary to vote for him!" He's possibly running these ads on my show, on stations that carry my show. This is profound. This is truly profound.

And the sad thing is, he appears to be right.