One more Democratic senator who has long been regarded as a safe prospect for reelection may be facing a challenging year in 2010. A new Rasmussen Reports telephone survey of likely voters in Wisconsin finds Republican Tommy Thompson edging incumbent Russ Feingold 47% to 43% in a hypothetical U.S. Senate match-up. Five percent (5%) like some other candidate, and four percent (4%) are undecided. Any incumbent who attracts less than 50% support at this point in a campaign is considered potentially vulnerable.