Nevada Sen. John Ensign (R) is expected to face a difficult primary challenge if he seeks reelection in 2012, but a new poll shows he may not be as vulnerable as some observers believe.

Ensign leads two of his potential primary rivals in head-to-head match-ups surveyed by Public Policy Polling, a Democratic firm based in North Carolina. Although against Rep. Dean Heller (R-Nev.), Ensign is below 50 percent — a warning sign for an incumbent.

The two-term senator leads Heller 45 to 37 percent. Against Lt. Gov. Brian Krolicki (R), Ensign's lead is 28 points — 55 to 27 percent. Krolicki opted not to run against Senate Majority Leader Harry Reid (Nev.) last cycle, presumably because he was contemplating a challenge to Ensign.

Moreover, 64 percent of GOP primary voters in Nevada approve of Ensign's job performance, while only 23 percent disapprove. His approval rating is even higher with conservatives — 71 percent back Ensign's performance.

PPP polled 400 Nevada Republican primary voters Oct. 7-9. The survey has a 4.9 percent margin of error.

Ensign is considered vulnerable, in part, because he's being investigated by the FBI and the Senate Ethics Committee over his efforts to get lobbying work for the embittered husband of his former mistress. Ensign helped Doug Hampton, the husband of Cynthia Hampton, get a job with a lobbying firm, and his parents provided the Hamptons with a payment of nearly $100,000, which they described as a gift.

Ensign has denied any wrongdoing.