National Party Leaders & Their Involvement in Partisan Primaries

Why are prominent national political figures involving themselves so heavily with the intra-party primary season this year? I can't remember this ever happening before, at least to this degree. Sarah Palin is flying around all over the country pulling for "her" candidates while rival Newt Gingrich is backing his choices, often in opposition to Palin's. In the Kentucky GOP senatorial primary, Minority Leader Mitch McConnell supported loser Trey Grayson while outgoing senator Jim Bunning strongly backed winner Rand Paul. In the Democratic side, we already witnessed the debacle in Pennsylvania, where the Obama administration actively tried to get incumbent Arlen Specter's opponent (and eventual winner) Joe Sestak to drop out of the race for the U.S. Senate. And here in Florida, Bill Clinton has flown down to campaign for Kendrick Meek against Jeff Greene. Why?

This all makes me a little bit uneasy somehow. It seems to me that the national figures should let their parties' state and local elections play out naturally. Only after a nominee has been chosen should they then get out and campaign against the "other side". I know that, being a Democrat, were I to decide to toss my hat into the ring for an elective office, it would really badly irk me to see Clinton, Obama, or another well-known figure visibly campaign for one of my opponents against me. Especially when I had grown to respect these people over the years for their sense of fairness and judgement. I imagine that the same is also true for those civic-minded Republicans running for office when they see party bigwigs stumping for their opponents. It is as if the parties are so heavily into top-to-bottom control that they feel that they can't allow the actual people to decide for themselves!