I wouldn't be surprised if younger voters make a radical shift rightwards soon, maybe not on social issues, but on fiscal issues for sure, so who knows, maybe Thune could do well among younger voters.
Fiscally conservative does not equal Republican. In fact, the last three Republican Presidents were the most fiscally irresponsible in our nation's history, despite all of them being self-declared fiscal conservatives. I'm sure many young people don't know that (even though I'm sure many of them remember Bush Jr.), but the Democrats can always remind them about this, especially if they nominate a candidate that communicates well with young voters like Obama.