The 2018 contests, particularly Senate and governors' races in Pennsylvania, Ohio, Michigan and Wisconsin, will help decide whether 2016 was a temporary setback for the Democratic Party's most powerful traditional ally or the turning point in a new chapter of declining relevance. Those states are home to more than 2.2 million of the 14.8 million union members nationwide, according to federal labor statistics.
The Pennsylvania election, which polls suggest is a toss-up, is an early test case.
Before the week's end, it will feature appearances from both Biden and Trump — two politicians who've recognized the sway of industrial unions and the white working-class voters they represent.
Trump, who won the district by 20 percentage points in 2016, is due to make his second stop on Republican candidate Rick Saccone's behalf Saturday, days after announcing a new steel tariff plan celebrated by industrial unions. Breaking from the party and declaring his determination to save the steel industry, Trump said he plans to impose steep new tariffs on steel imports and 10 percent on aluminum imports.
There are some 17,000 steelworkers in the Pittsburgh-area district. Labor leaders estimate the total union household vote in the district at 87,000 or about one-fifth of the registered electorate.
On Trump, there was little disagreement among the scores of union members who gathered to see Biden and Lamb on Tuesday: In more than two dozen interviews, not a single person would admit to voting for the Republican president.
"Nobody's gonna admit it — maybe if you get a few beers in them," said Nick Lombardo, a 33-year-old carpenter from Pittsburgh.
"I hate him," said Lombardo's friend, 28-year-old Justin Shook of Butler, Pennsylvania.
But Trump's tariff plan?
"That's the one thing I agree with Trump on so far," Shook said. "It's only going to be good for us."
Rick Bloomingdale, the Pennsylvania AFL-CIO president, suggested that Trump's move on tariffs wouldn't sway voters, despite his success in the region and speculation that he timed his announcement to help Saccone.
"Our people who liked Trump before still do," Bloomingdale said. "The ones who didn't, still won't. And there will be ticket splitters in both camps."
A Lamb victory would signal a shift back toward the region's historical partisan leanings.
Biden and his former boss, President Barack Obama, won union households by 20 percentage points in 2008 and 18 in 2012, margins that helped them twice sweep the band of states from Pennsylvania to Wisconsin.
Trump cut that gap to 8 percentage points, according to 2016 exit polls, the smallest margin for a Republican since President Ronald Reagan's landslide re-election in 1984 and enough to move those key Great Lakes states into the GOP column.
Trump's tariff decision has also scrambled political alliances on Capitol Hill, where his free-trade party is pushing him to back down. Meanwhile, some Democrats, including Ohio Sen. Sherrod Brown and Pennsylvania Sen. Bob Casey, applauded the move.
Trump's announcement allows Democrats like Brown "a real opportunity ... to highlight what they've been talking about for decades" and remind union members who support Trump that they can still vote for Democrats elsewhere on their ballots, said Brown's campaign manager, Justin Barasky.
In Pennsylvania's 18th Congressional District, support for labor and trade policy was a fight well before last week's tariff decision.
Lamb secured labor backing early, shifting the dynamic in a district held previously by Tim Murphy, the rare labor-friendly Republican in Washington. Murphy's standing with unions was so strong that Democrats didn't even field a candidate against him in 2016.
But as a state legislator, Saccone backed proposals to block the state from collecting dues via public sector union members' payroll. He's opposed requiring union work on public infrastructure projects and mandates that the projects use domestic materials. The vote has opened him up to fierce criticism from Lamb.
"If you want to represent the people who live here, you have to stand up for them," Lamb declared.
Saccone calls his Democratic opponent's characterization "ridiculous." He doesn't dispute his voting record in Harrisburg, however, telling The Associated Press he believes "competition" strengthens the market. He said union members know his agenda — "fighting to beat back regulations" and "cutting taxes" — saves jobs.
"Their union leadership doesn't represent them," Saccone said. He added that rank-and-file laborers are overwhelmingly opposed to abortion and gun restrictions, putting them at odds with the Democratic Party platform.
On tariffs, the two men are largely in agreement. Each endorsed the president's tariffs generally in a recent debate, noting that the White House hadn't ironed out details.
Saccone's position puts him at odds with the GOP leadership that is bankrolling much of his campaign effort. Lamb is walking a tightrope similar to other Democrats who align themselves with unions but advocate for international trade.
The Democrat argued in a previous AP interview, before Trump's announcement, that tariffs are essentially reactionary, isolationist moves that don't necessarily help workers. He charged that corporate tax policy is more to blame for U.S. job losses than trade policy.
"What I feel like we are seeing are unilateral moves, and it's hard for me to see the strategy behind it," Lamb said. "I haven't heard a great explanation for them as to why they think these tariffs are going to benefit us."
Back at the Pittsburgh union hall, Biden was working to keep his eye on the bigger prize: a renewed alliance between a unified labor movement and Democrats in 2018.
"You guys have always had my back," he said. "Now go out and make sure that they win."
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Follow Bill Barrow on Twitter at https://www.twitter.com/BillBarrowAP .
