Senate Minority Leader Mitch McConnell is quietly maneuvering to field a slate of GOP Senate candidates in critical battleground states, attempting to avoid a repeat of election cycles a decade ago when candidates emerged from primaries only to implode and deny his party the chance to take back the majority. This time around, the wily Kentucky Republican has a high-spending super PAC and he’s prepared to use it. With 20 GOP seats to defend — including five open seats where Republican senators are retiring — compared with 14 seats that Democrats have to defend, McConnell is making clear he’s not...