DAVENPORT, Iowa — Returning to Iowa where the latest polls show his campaign suddenly slipping, Newt Gingrich lamented what he called a barrage of negative ads aimed at him and promised a 44-stop “jobs and prosperity” tour before the voting on Jan. 3 to fight back. Mr. Gingrich renewed a vow to stick to the high road and campaign positively, but he still aimed a stinging barb at unnamed Republican rivals for ads that have relentlessly gone after his record on Freddie Mac and other issues and seem to be raising doubts with voters. “I’m candidly very disappointed that some...