The special-election primary to replace resigning Oklahoma Sen. Tom Coburn is tightening after an advertising blitz benefiting Republican state Speaker T.W. Shannon. A poll conducted by Public Opinion Strategies on behalf of Oklahomans for a Conservative Future, an outside group backing Shannon, found the state speaker cutting into the early GOP primary lead of Rep. James Lankford. Lankford, who is the favorite to fill the remaining two years of Coburn’s term, led with 37 percent, followed by Shannon with 28 percent and former state Sen. Randy Brogdon with 7 percent. A half dozen candidates are running to replace Coburn, a...