Why is Joe Lieberman speaking at the Republican convention tonight? Simple. John McCain asked him to. Lieberman, the independent Democratic senator from Connecticut, long ago endorsed McCain for the presidency over his own party's nominee, Barack Obama. But speaking to thousands of excited Republicans in St. Paul - he's never been to Republican convention before - was still a big step for Lieberman. He agreed to address the convention knowing full well that his appearance may be used against him by partisan Senate Democrats, some of whom already want to strip him of his chairmanship of the homeland security committee. Lieberman has a target audience, but it's not the delegates or Republicans in general. It's independents and soft Democrats. Lieberman is popular with this constituency. So is McCain. But because the Republican party has shrunk since 2004, McCain must do especially well with independents and wandering Democrats if he's going to win. Yes, he has to mobilize the Republican base as well. But he has a separate ally to help activate the base. That's Sarah Palin.