LANSING, Mich. (AP) — In 2016, Donald Trump tore down Democrats' "blue wall," winning the White House with surprise victories in Michigan, Pennsylvania and Wisconsin.

This year, Joe Biden is trying to rebuild.

The Democratic presidential nominee's first pandemic-era campaign trips beyond his home in Delaware are taking him to all three states, an indication of how closely Biden's electoral prospects are tied to his ability to flip those political battlegrounds.

Last week, Biden traveled to Wisconsin and was followed quickly by running mate Kamala Harris, who held her own events there on Labor Day. On Wednesday, Biden heads to Michigan to tout a plan for boosting U.S. manufacturing. He also has two stops scheduled this week in Pennsylvania.

Though the Biden campaign often emphasizes that it sees multiple ways to secure the 270 Electoral College votes it needs to win in November, the quickest path runs through Michigan, Pennsylvania and Wisconsin.

"If Biden wins any of them — but particularly any two, with some of the other states that are in play — it's pretty impossible for Trump to win the Electoral College," said veteran Democratic strategist Joe Trippi. "It makes tremendous sense to make those three states the base foundation of any strategy to win. If Biden wins all three, it's over."

Biden's aides believe his focus on the economy and Trump's handling of the coronavirus pandemic will resonate with key voters nationwide but particularly in states like Michigan, which took one of the sharpest hits nationally from the pandemic.

The unemployment rate in the state spiked at 24% in April, according to the Bureau of Labor Statistics. The statewide rate has since recovered to 8.7%, but Michigan has nearly 414,500 fewer jobs than it did when Trump was inaugurated.

Trump supporters say he has fulfilled his promise of creating jobs and was temporarily sidetracked by the pandemic. But the jobs numbers show that hiring at factories across the Midwest — including in Michigan, Ohio and Wisconsin — began to stall and then decline in the summer of 2019. The Biden campaign sees an opening there to argue that, even before the pandemic, the president failed some of his most loyal blue-collar backers.

Trump won Michigan by the narrowest margin of any state in 2016 — fewer than 11,000 votes — and Democrats made huge gains there in the midterms, winning every major statewide office and a handful of federal seats as well. Polls conducted earlier in the summer suggested a Biden advantage there, though no polling has been conducted since the August party conventions. Polls conducted in Michigan throughout the 2016 campaign showed Hillary Clinton with a lead before Trump went on to win.

Democrats believe Biden has a particular appeal to the white working-class voters in Michigan, as well as in Wisconsin and Pennsylvania, that swung to Republicans in 2016 and could help tip the scales in a narrow race in 2020.

They also see reasons for optimism in Democratic gains during the 2018 midterms in all three states, which were powered in part by an exodus of suburban women from the GOP. And they believe that a stronger emphasis on minority turnout — with Harris, the first African American woman on a major ticket, focused heavily on Black voters in key states — will help Biden make up some of the ground Clinton lost in 2016.

Democratic operatives in Michigan say it seems that Biden has indeed learned some lessons from Clinton's defeat.

"The campaign's taking it more seriously from the start than national Democrats did four years ago," said Amy Chapman, who worked as Barack Obama's Michigan state director in 2008. "They started doing advertising earlier than they did last cycle — last cycle they were only up at the very end — and the ads show what Biden would do, as well as showing a contrast with Trump."

The Biden campaign is heavily outspending the Trump campaign on-air in all three of the key battleground states. Since Biden became the presumptive Democratic nominee in early April, his campaign has spent about $59.8 million to the Trump campaign's nearly $26.8 million across all three states, according to the ad tracking firm Kantar/CMAG. The difference is starkest in Michigan, where the Biden campaign has spent $17.2 million to Trump's $6.7 million.

For future spending, however, the two come about even, with Biden reserving $33.5 million on air and Trump reserving $32.7 million across all three states. Trump is slightly outspending Biden in upcoming ad reservations in Pennsylvania, while Biden is slightly outspending the president on air in Michigan.

Trump's campaign also plans to blanket the Midwest in the crucial final stretch.

Vice President Mike Pence campaigned in northern Michigan last month and presidential daughter and adviser Ivanka Trump visited General Motors in Warren, Michigan, last week. Wisconsin has seen a flurry of activity in recent weeks as well, with visits from Trump and Pence over the past month.

Trump and Biden will both be in Pennsylvania on Friday at a Sept. 11 memorial in Shanksville, the site of the 2001 crash of United Flight 93. Trump and Pence are both expected back in the state in September, and female Trump family members, including Ivanka Trump, are visiting Philadelphia's heavily populated suburbs to appeal to women there.

Republicans also argue that a robust, in-person field operation in the Midwestern battlegrounds gives them an advantage over Democrats, who are still doing most of their organizing online out of concern for public safety during the pandemic.

"We're door-knocking all over the place, our local candidates are door-knocking and the Trump campaign volunteers are door-knocking," said Elizabeth Preate-Havey, GOP chairwoman in Montgomery County, Pennsylvania's third-most populous county.

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Jaffe reported from Wilmington, Delaware. Associated Press writers Marc Levy in Harrisburg, Pennsylvania; Thomas Beaumont in Des Moines, Iowa; and Josh Boak in Washington contributed to this report.