From the Review-Journal:
The Reid campaign also contends the Mason-Dixon poll isn't a true ballot test because it doesn't include the full general election slate of eight candidates, including the Democratic senator who faces little-known primary opposition, the GOP nominee, several nonpartisan candidates and one each from the Independent American Party and the Tea Party of Nevada, and "none of these candidates."
Reid's unfavorable rating is at 56%, according to this poll. We haven't gotten a response from the Reid camp yet, but it appears the thinking is that the poll numbers aren't valid because they don't take into account the fact that voters could choose many other candidates -- or no candidate at all -- and not just face a choice between Reid and his Republican opponent.
Reid's campaign also maintains that internal polls show the senator ahead.