Minutes after U.S. Sen. Al Franken announced his resignation, Gov. Mark Dayton said he was not yet ready to announce his pick to fill Franken's seat.

But the DFL governor said he planned to make his decision within days — and many at the state Capitol expect he'll appoint Lt. Gov. Tina Smith. A high-ranking Democratic source told the Star Tribune on Wednesday that Smith, a close ally to Dayton and longtime DFL insider, is his likeliest choice to replace Franken. Under that scenario, Smith would serve as a temporary replacement who would not run for the seat in a November 2018 special election.

"Events have unfolded quickly; thus, I have not yet decided on my appointment to fill this upcoming vacancy," Dayton said in a statement. "I expect to make and announce my decision in the next couple of days."

The governor said he would not comment further on the selection until he formally announces his pick. If he does select Smith, her move to Washington, D.C., would prompt a cascade of shifts in state politics — and help set up Minnesota's elections next year as among the most costly and closely watched in the nation.

"Minnesota is going to be the epicenter of politics next year," DFL Chairman Ken Martin said Wednesday.

Even before Franken formally resigned, political operatives were scrambling to compile research on the large and growing number of potential candidates for the seat. Following the governor's short-term appointment, a special election next November would let voters choose who fills out the remaining two years of Franken's term.

That would make an already busy election year in Minnesota even busier. Dayton is retiring, leaving next year's gubernatorial race wide open. U.S. Sen. Amy Klobuchar, also a Democrat, is running for re-election. And the state is likely to see as many as five ­competitive U.S. House races, which could help determine which party holds the majority.

Ambitious elected officials are already mulling the possibility of another high-profile political opening. State Sen. Melisa Franzen, DFL-Edina, said the party needs new faces.

"It's our generation's time to lead, and we need the fresh perspective on issues because we're being burdened by the decisions being made by people who have been in Congress for 50 years," said Franzen, who held her newborn baby on the floor of the Senate earlier this year.

If he taps Smith as a caretaker senator, Dayton would be giving the job to someone who apparently has no further political ambitions. Smith was Dayton's chief of staff in his first term, after which he chose her as his second-term running mate. She had previously decided not to run for governor to replace him.

"The governor came to rely on her judgment and advice while she was chief of staff," said Ryan Winkler, a former DFL legislator who is running for attorney general.

Dayton is being widely advised to select a woman, given the circumstances of Franken's departure. More than half a dozen women have alleged that he kissed or grabbed them without permission.

"I feel in this environment, and given what's happening ... a woman would be very appropriate," said Rep. Frank Hornstein, a longtime progressive activist.

If Smith is appointed, however, state Sen. Michelle Fischbach, R-Paynesville, the president of the Senate, would become the lieutenant governor.

That in turn would set up a special election for Fischbach's Senate seat along with the seat being vacated by Sen. Dan Schoen, DFL-St. Paul Park, who recently announced his own resignation after allegations of ­sexual harassment.

The state Senate is currently controlled by Republicans by one vote, which means the DFL could take control if it holds the Schoen seat and flips the Fischbach district.

Meanwhile, a variety of party big names would likely consider running for the U.S. Senate seat now held by Franken: U.S. Rep. Keith Ellison, who would be the state's first black senator; two other U.S. representatives, Rep. Betty McCollum and Rep. Rick Nolan; or state Rep. Ilhan Omar, who would become the Senate's first Somali-American member.

Several current candidates for governor might also reconsider and run for Franken's seat, including U.S. Rep. Tim Walz or St. Paul Mayor Chris Coleman. Attorney General Lori Swanson, also widely believed to be considering a run for governor, could run for the Senate instead.

Republicans would also try to mount a serious candidate, having not won a statewide race since Tim Pawlenty was re-elected governor in 2006.

David FitzSimmons, a former Republican legislator who is now chief of staff to U.S. Rep. Tom Emmer, said that although a Franken resignation would open up the Senate seat, the DFL would no longer have to defend him.

"So, in some ways it's to their benefit that he goes," he said.

Pawlenty would be at the top of any list of potential candidates.

Former Sen. Norm Coleman, whom Franken beat in their epic 2008 battle, is another possibility, as is Emmer or U.S. Rep. Erik Paulsen. State Senate Majority Leader Paul Gazelka and House Speaker Kurt Daudt both have rising profiles.

Each side would have to raise $15 million to $20 million to compete for the open Senate seat, and do so quickly.

J. Patrick Coolican • 651-925-5042