ERIE, Pa. — Midway through another raucous arena rally last week, President Trump offered a revealing aside about Rep. Lou Barletta, the Republican he recruited to jump into Pennsylvania’s Senate race.

“I got him into this,” Trump said, musing about what he called “the only bad thing” about Barletta’s candidacy before thousands of supporters resplendent in red ball caps. “For the rest of his life, he could have been a congressman.”

A moment later, any fleeting dismay about the prospect of Barletta’s looming unemployment had passed, but the riff was telling. Polling shows Barletta well behind Democratic Sen. Robert P. Casey Jr. Indeed, it has become increasingly hard for Republicans to remain optimistic about the chances for him and other GOP candidates across the industrial Midwest.

A number of Republicans running for governor or senator in Wisconsin, Michigan, Ohio and Pennsylvania, including several who hitched their wagon to Trump’s political movement, are behind in polls by double digits, a remarkable turnabout in swing states that were key to the president’s 2016 victory.

Just ahead of his departure for a rally in Richmond, Ky., President Trump spoke about the anticipated size of the crowd. (The Washington Post)

If current polling averages hold, Democrats will maintain all their Senate seats in those states, pick up a handful of House seats and, in some cases, retake the governors’ mansions. In nearby Iowa, a state Trump won by nearly 10 points, the Democratic candidate for governor was running about even with the Republican governor in a Des Moines Register/Mediacom Iowa Poll. Polling this week found Gov. Scott Walker (R-Wis.) trailing his Democratic opponent, Tony Evers.

The dramatic shift has forced political strategists to reevaluate their post-mortem lessons from the 2016 election, while raising new questions about Trump’s staying power in 2020. Democratic strategists, who worried that Iowa and Ohio were slipping away from them in presidential years, are now heartened and have begun to return their attention to the traditional bellwethers.

“One false assumption that was made was that a Trump voter from the 2016 election was necessarily a Republican voter,” said John Brabender, a GOP consultant who is working with Barletta. “We forget about the power of Hillary Clinton being on the ballot in 2016. If Hillary was on the ballot, Republicans would probably be doing better in all of these states.”

There is a clear historical precedent for such a shift. Then-candidate Barack Obama swept the industrial Midwest in the 2008 elections, only to find his party battered in his first midterm contest two years later, when Republicans retook governorships in Ohio, Michigan, Iowa and Wisconsin, along with Senate seats in Indiana and Wisconsin. Obama was nonetheless able to come back and win those same states, with the exception of Indiana, in his 2012 reelection.

Pollsters do not rule out Trump repeating that success in 2020, especially if the economy remains strong. “He could certainly do what Obama did,” said Berwood Yost, the polling director at Franklin and Marshall College, which tracks Pennsylvania voters. “Trump’s approval rating in our state is about the same place Obama’s was in 2010.”

Still, the short-term impact is dire for Republicans. After surprising the nation in 2016, Trump appears to be driving turnout this year that will largely benefit Democrats, as moderate voters, and college-educated women in particular, seek an outlet for their frustration with his policies and behavior. Trump’s aggressive campaign schedule for Republicans in these states has so far failed to turn the tide.

“They thought they had unlocked some formula that would make them successful. But it was only Trump and only that year,” Sen. Brian Schatz (D-Hawaii) said of the 2016 election. “What the Republicans are doing now isn’t working for union members or struggling families. It’s not working for young people. It’s just not working.”

Sen. Tammy Baldwin (D-Wis.), who began the year as a leading target for conservative super PACs but is besting Republican challenger Leah Vukmir by about 10 points in recent polls, attributes her success to the return of an energized Democratic voting base, driven by issues such as health care and sustained by how the party, in her view, has built a case that’s bigger than just opposing Trump.

“The story in Wisconsin in ’16 was actually a drop in voter participation that was unanticipated,” Baldwin said. “I have seen in the past two years, especially among issues that are deeply personal like health care, people realizing what’s at stake and they’ve been active and organizing. They are saying, ‘No more sitting on the sidelines.’ ”

That same pattern is playing out in Michigan, where Sen. Debbie Stabenow (D) and the Democratic candidate for governor, Gretchen Whitmer, have both had comfortable margins in recent polling. Trump won the state by a whisker-thin margin of 10,704 votes in 2016.

“Everything I am seeing in my numbers is revolving not around his job approval but whether you view him favorably or unfavorably,” said pollster Richard Czuba, who runs a statewide survey for the Detroit News and WDIV. “Donald Trump doesn’t have an opponent, and that is his problem right now. ”

The result is a sharp overall surge in voter enthusiasm in the state compared to 2016, and big swings in suburban areas such as Oakland County, the state’s wealthiest region, outside Detroit. “We are finding it difficult to find college-educated women in Oakland County who will call themselves Republicans,” Czuba said.

That has created reverberations in the race to succeed retiring Rep. Dave Trott (R) in Michigan’s 11th District, which includes parts of the county. The district, which was drawn in 2011 to ensure Republican victories, voted for Trump in 2016 by more than four points, but recent polls have shown Democratic candidate Haley Stevens, a former Obama administration official, in the lead.

Her Republican opponent, Lena Epstein, co-chaired Trump’s 2016 Michigan campaign and began the election cycle calling herself a surrogate for Trump, appearing on Fox News to praise the president’s “abandonment of political correctness.” She has since refocused her candidacy, dodging questions about her past commitment to join the conservative Freedom Caucus and casting herself as a bipartisan unifier.

“People don’t always respect a woman in business,” Epstein says in her most recent ad, which seeks to ride the coattails of the #MeToo movement. “I’ve been underestimated, talked down to and dismissed.”

John Yob, a consultant for Epstein, says the recent confirmation of Justice Brett M. Kavanaugh to the Supreme Court has provided a late boost to the campaign, a trend that other Republicans hope could turn these races in the coming weeks.

“We have seen a significant six-point jump in polling since the Kavanaugh confirmation and would welcome the president to campaign for Lena anytime,” Yob said, describing the campaign’s private data.

But the Kavanaugh effect apparent in other parts of the country may not be enough to swing races here. In many of the Great Lakes states, candidates like Barletta who most tied themselves to the Trump agenda are still flailing. In Ohio, Rep. James B. Renacci (R), whose first Senate campaign ad was about his tight bond with Trump, has yet to come within 10 points of incumbent Sen. Sherrod Brown (D-Ohio) in a major public poll.

“We have lost millions of members of our party in the last year,” said John Weaver, a Republican adviser to Ohio Gov. John Kasich and a Trump critic, reflecting on how Trump’s bid split the party. “A MAGA candidate who runs as a junior member of the walking dead and wins the primary is going to find themselves shot in the general election.”

Complicating things further is the devotion of the Republican base to Trump’s take-no-prisoners approach, which can make it dangerous for GOP candidates who seek to create some distance.

“When you talk to Republican primary voters, the number one issue is, ‘Are you with Trump?’ ” said retiring Sen. Jeff Flake (R-Ariz.), a Trump critic. “So, once you get through that, it’s tough to pivot.”

Trump’s decision to renegotiate trade agreements with Mexico and Canada, and to start an escalating tariff war with China, have muddled the political fallout in the Midwest, even though the economic effects have been relatively pronounced. Rising steel and aluminum prices, falling soybean prices, and new restrictions on car imports have sparked a wave of headlines in the region about layoffs and struggling farmers.

But Democrats in the region have largely taken a nuanced approach to the same issues, with many candidates praising Trump’s efforts to renegotiate the North American Free Trade Agreement. Brown, the Ohio Democrat who opposed NAFTA and other trade deals, has praised Trump’s approach.

“Actually, that’s one of the things I have more in common with Trump than many Republicans,” Baldwin said with a chuckle when asked about her protectionist tilt on trade issues, which she said has countered the Republican case against her.

Rep. Tim Ryan (D-Ohio), a potential long-shot challenger of Trump in the 2020 presidential election, said the real driving force in this part of the country comes from a sense that the region is still not benefiting from the nation’s overall economic growth.

Trump was able to win in 2016 by contrasting himself with Clinton, who was boasting of an economic resurgence under Obama, in the stock market and unemployment rate, that many voters did not feel in their daily lives.

“Now he is falling into that same line of argument and people are saying, ‘Not so much,’ ” Ryan said. “There is no substantial change.”