I don't have any first hand experience with any of the others, but Pryor got 80% of the vote in 2008. He wasn't even challenged by a Republican, the Green party got the other 20% of the vote.
That was 2008 when the GOP was at a low point nationally. The GOP is certain to run someone in 2014, and Arkansas is quickly becoming a solid blue state. That Green party vote in 2008 was almost all votes for someone who wasn't a Democrat, not votes for the Green party.
As for Warner, he was running against someone from the right edge of the GOP. If the GOP runs someone better and 2014 is a better year over all for the GOP, Warner could be vulnerable.