PORTSMOUTH, N.H. – His campaign in difficult straits after disappointing showings on the stump and the debate stage, former Florida governor Jeb Bush insisted Thursday that he has the skills and the staying power to withstand a downturn. “It’s not on life support,†he said of the once-formidable campaign operation he has been forced to trim back as a cost-saving measure. “We have the most money, we have the greatest organization,†Bush told reporters following a brief campaign stop here. “We’re doing fine.â€