[What to expect at Trump’s 2020 State of the Union Address.]

LANSING, Mich. — Gretchen Whitmer has only been Michigan’s governor for 13 months. She has yet to “fix the damn roads,” as she repeatedly promised on the campaign trail. Beyond her state’s borders, she is largely unknown.

But Ms. Whitmer, who on Tuesday will give the Democratic response to the State of the Union, has something her party covets: proof that Democrats can still win in the Midwest. And those swing-state credentials help explain why Speaker Nancy Pelosi called to offer her a national audience. “She’s fresh, she’s smart, she’s witty, she’s bold — and she comes from a state that the Democrats need,” said Kym Worthy, a Democrat who is the elected prosecutor in Wayne County, which includes Detroit.

That Democratic leaders in Congress looked beyond their chambers to select Ms. Whitmer for the speech underscores Michigan’s potentially decisive role in this year’s presidential race. Just four years ago, Democrats were confident they would win the state, and many of them dismissed Donald J. Trump’s frequent campaign visits as quixotic. But after Mr. Trump took the White House by narrowly carrying Michigan, along with Wisconsin and Pennsylvania, a Democratic reckoning ensued.

The party’s answer to that reckoning was Ms. Whitmer, an ex-prosecutor and longtime state lawmaker who won the governor’s office in 2018 by 10 percentage points. Her victory ended full Republican control of Michigan’s capital and lifted Democrats’ hopes of reclaiming the state — and with it the presidency — in 2020.