I agree with Lunar. A tough primary isn't a bad thing, just ask Obama. He was able to paint himself as a moderate by beating Clinton who was perceived to be a liberal.
Whoah, woah. He managed to paint himself as an anti-establishment person by beating Clinton, not necessarily a moderate... perhaps a tangent, and perhaps an even more important issue, but Obama's alleged [?] moderate-status didn't come from beating Clinton. They were basically identical on 99% of the issues.
He ran to Clinton's left on a number of issues, such as diplomacy (meeting foreign leaders) as well as the generic value of change. I imagine that most of the hard-core liberals backed Obama during the primary, so this isn't exactly what I was getting at. Clinton herself seized this weird mantel of being the working-class, blue-collar hero that probably wouldn't have survived the general election, but there you go. Obama was the wine-drinker and Clinton was the one drinking whiskey and chasing it with beer.
Well he had to be perceived as some type of moderate in order to get 20% of the conservative vote in the election...