Story highlights Democratic Gov. Mark Dayton would appoint a replacement if Franken resigns

That would set up a special election in November 2018

(CNN) Should Sen. Al Franken decide to step down, his resignation would set up a gubernatorial appointment and open up a new Senate battleground in 2018.

Minnesota Gov. Mark Dayton does not plan to get ahead of Franken's scheduled announcement Thursday, a senior Minnesota Democrat close to Dayton told CNN, but the governor's "expectation and hope is for Franken to resign."

Should Franken step down, top names to replace him are Democratic Reps. Keith Ellison and Tim Walz, this official said. Another leading contender will be Lt. Gov. Tina Smith, a former chief of staff to Dayton.

"Don't overlook Lt. Governor Smith," the official said. "She could be the perfect choice."

Dayton, a former US senator, might also tap his former colleagues for advice in his pick, including Senate Minority Leader Chuck Schumer. "There will be an open line of communication," said the senior Democratic strategist.

Read More