Opponents and their researchers have begun working — mostly behind the scenes — to highlight perceived soft spots in his conservative bona fides.

And Thompson will have to neutralize questions on the campaign trail and in the media about his centrist votes in the Senate, his stances on litmus test conservative issues including abortion and — perhaps most significantly — his work as a lawyer and lobbyist.

Thompson's biggest challenge will likely be cementing his image as a conservative country lawyer fixin' to shake up Washington — before his opponents brand him as an influence peddler and trial lawyer.

— Lobbyist: Thompson made nearly $1.3 million over about two decades of lobbying both before and after his eight-year Senate stint, according to government documents and media accounts from his successful run for the Senate in 1994.

— Trial lawyer: Before Thompson won his Senate seat, published reports said his private law practice handled personal injury cases and defended people accused of white-collar crimes. And in the Senate, he opposed some legislation intended to rein in escalating jury verdicts and attorneys' fees.

— Campaign finance reformer: Thompson was among the leading Republican backers of the sweeping package of campaign finance reforms commonly known as McCain-Feingold.

— Centrist senator: Though the influential American Conservative Union scored Thompson's Senate career voting record at 86 percent, some of the votes on which he strayed from the GOP fold could prove problematic for ardent partisans.

— Abortion-rights supporter: Every time Thompson got the chance in the Senate, he voted with those who oppose abortion rights. But the social conservatives for whom abortion is a litmus test scrutinize every bit of a politician's record — and Thompson's provides some fodder for opponents to question the depth of his opposition to abortion rights.

On candidate surveys in 1994 and 1996, he answered that he favored abortion always being legal in the first trimester of pregnancy and opposed an amendment to the U.S. Constitution "protecting the sanctity of human life."