Honestly, if I was a Republican I wouldn't be worried that a bad nominee would hand Obama a second term. With the exception of Bachmann, none of the people with any chance of winning the nomination are really handicapped in the general election.
Like most re-election campaigns, it's mostly going to be about the economy. If the economy gets better, there isn't anyone the GOP could nominate that would win. If it gets worse, they can't lose. Even if it keeps muddling along, the election will probably turn on other issues.