That's a vague statement...more elaboration is a required. Is it your judgment alone or do you have facts?
I have watched Duncan on film and live. He has a command of the facts and the issues that he is strongest in. But he plays too few of them. Watching his debates and such it is China and Immigration almost all the time, with little Iraq tossed in. Important issues yes indeed. But this is not a congressional run, and he sounds like he is running in his district not nation wide. The guys in the big show have to play to a much larger audience than the core of staunch Conservatives. There are fiscal conservatives, social conservatives and the like, plus those in the middle we have to woo. Also there are dozens of issues that concern them.
If the election was just Free Republic or for conservative of the year, Duncan could score that in a heartbeat, but we are too few in number to make that translate into a national win. We have to win over the middle, and they do not want to be preached to, they want to be sold. Duncan is a great conservative preacher, but his sales skills need refining.
Another thing, while I know he has put out statements on his web site and such on a number of issues, he has to actually play on all those issues publicly as much as possible.
Also I have said often that this is a political game and after 27 years in the house Duncan has not really stood out. It is past the time that most politicians who end up as national players have broken out. I am told it is because he "does not bow down" or "does not play politics". Well guess what, the game the left and the middle have brought us is politics, so to win it we have to be in it. Otherwise we are just those whiny folks over on the fringe arguing with the wind.
As far as evidence, it is all simple observation. Where has he been the past 27 years, why has he not gained more notoriety and where is he in the polls after a year of being in the race. Blame the press or whatever, but a politician is responsible for his own destiny as much as anyone. To try and pass it off as some conspiracy or the like does him and his efforts a disservice, because it grants a scapegoat, not a solution, which in the end is what you are after, getting it fixed and moving up to victory.