WASHINGTON — In the year since he stepped down from Congress, Steven C. LaTourette, a Republican from Ohio, has emerged as one of the top generals in the establishment Republicans’ war against the Tea Party. It is a role that has benefited the Main Street Partnership, a corporate-backed advocacy group he runs, and its effort to raise millions of dollars to protect centrist Republicans from Tea Party challengers. It has also helped draw clients to a separate lobbying office Mr. LaTourette and his wife have set up across the street from the Capitol. But this blitz of activity has led...