Gov. Mark Dayton will appoint Lt. Gov. Tina Smith to replace Sen. Al Franken as Minnesota’s next U.S. senator, according to two high-level Democratic sources with knowledge of the decision.

Dayton will name Smith as his choice at a Wednesday morning news conference, and she plans to run for the seat in a 2018 special election, the sources said.

Franken announced last week his intention to resign after he was accused of improper conduct toward more than half a dozen women.

In selecting Smith, the governor is choosing one of his most trusted advisers and someone who has worked for years traveling the state and building relationships with influential DFLers and business leaders.

Smith was Dayton’s first chief-of-staff after careers at General Mills, Planned Parenthood and the city of Minneapolis, where she was chief-of-staff to former Mayor R.T. Rybak.

“She’s a person of intelligence and competence, and from what I’ve seen, she’d be a good campaigner and a good candidate,” said state Sen. Richard Cohen, DFL-St. Paul.

Gov. Mark Dayton will announce his pick to replace Sen. Al Franken during a 10 a.m. news conference Wednesday.

The selection also means Minnesota will have two female U.S. senators for the first time in history.

Dayton’s pick will draw national interest given the stakes: Open seats in the upper chamber are rare. Alabama voters went to the polls Tuesday and elected Doug Jones, putting Democrats in closer striking distance to the majority, which Republicans now control 51-49. When this edition went to press, Republican candidate Roy Moore still had not conceded to Jones.

But next year’s overall Senate map favors Republicans. Democrats will defend 26 seats with Franken’s departure, whereas Republicans will defend nine seats.

The race for Franken’s seat, which will cost the two sides tens of millions of dollars, will make 2018 one of the most pivotal Minnesota elections in years.

In addition to two Senate races — one to fill the Franken seat plus Sen. Amy Klobuchar’s re-election campaign — there will be as many as five competitive U.S. House races in Minnesota.

An open governor’s race puts Republicans in a position to take full control of state government for the first time in half a century.

The special election is expected to draw contenders on both sides. Even with Smith’s selection, DFL U.S. Rep. Keith Ellison is mulling the race, a source close to Ellison said last week. Former GOP Gov. Tim Pawlenty is being wooed by Republicans.

The winner of the 2018 election will be expected to run again for the full six-year term in 2020.

Smith, 59, lives in southwest Minneapolis with her husband, Archie, an investment manager. They have two adult sons. She moved to Minnesota in the 1980s for a marketing job at General Mills and later founded a marketing and public relations firm. That led to work on a series of DFL campaigns, including as an adviser to Walter Mondale’s last-minute campaign for U.S. Senate in 2002 following the death of Paul Wellstone.

The following year, Smith was recruited to become vice president of public affairs for Planned Parenthood Minnesota, North Dakota, South Dakota.

Smith left Planned Parenthood in 2006 to take over as chief of staff for then-Minneapolis Mayor R.T. Rybak. Rybak competed in the 2010 race for governor, tapping Smith to run his campaign.

When Dayton emerged victorious, he recruited Smith to join his campaign on the advice of his ex-wife, Alida Messinger, a major Democratic donor.

In recent years, Smith has become nearly as much the face of the administration as Dayton himself and has transformed the often obscure role of lieutenant governor.

For a time, Smith’s growing profile led to speculation that she would run for governor herself. Instead, she decided not to join the field and focus on closing out the final year of Dayton’s term.

The Dayton appointment will signal the closing chapter in the political career of Franken, who until a few weeks ago had transformed his comedy career into political stature — he was a powerhouse fundraiser and sought-after surrogate — but is leaving the U.S. Senate at the urging of his Democratic colleagues.

Franken’s office said that a final resignation date has not been set.

Gov. Mark Dayton will announce his choice to replace Sen. Al Franken at a Wednesday news conference.

Staff writer Jennifer Brooks and Erin Golden contributed to this story.