The high-profile involvement of S.E.I.U., whose membership in the Midwest has steadily increased over much of the past generation, is in some sense a reflection of the changing economic landscape of the region, where health care has replaced manufacturing as the top employer in many states.
But in another way, the effort is a matter of survival: with industrial unions depleted by globalization and automation, the stronger position of public- and service-sector unions has made them bigger targets for the right. Several Republican governors elected since 2010 embraced legislation that would restrict what public-sector unions can bargain for or would rein in their members’ pensions.
Recent polling suggests a climate in which Democratic candidates may regain momentum. A recent NBC News/Marist poll showed Mr. Trump with a net negative approval rating of roughly 20 percentage points in the three typically Democratic states he carried. The Republican governors of Illinois, Michigan and Wisconsin also appear to be unpopular, though term limits prevent the Michigan governor from running again.
Some public opinion experts believe that Republicans have overreached on the narrower issue of unions in particular.
“In Midwestern states, the battlegrounds, people really like unions,” said Celinda Lake, a Democratic pollster who has done work for unions, including the service employees. “That doesn’t mean they’re uncritical, but they like them, they cannot imagine a world without unions.”
But Patrick Semmens, a spokesman for the National Right to Work Committee, a group that has frequently arrayed against unions in legislative fights, argued that the issue was more complicated, pointing to polls showing that even pro-union voters tend to favor laws that allow workers to opt out of union dues.
“If the issue Big Labor plans on raising is right to work, then that should be a winning debate for candidates who have voted to oppose forced union dues,” Mr. Semmens said in an email.
In a similar vein, some union leaders have conceded that many of their members voted for Mr. Trump, whose margin of defeat among union households was the narrowest for a Republican nominee since Ronald Reagan.
One of the S.E.I.U.’s biggest initiatives in recent years has been the Fight for 15 campaign, an effort to raise wages and win union rights for low-paid workers that began in the fast-food industry and has helped win minimum wage increases across the country. Mr. Courtney, an architect of that effort, said the Fight for 15 would join the new initiative and that the idea was to combine two projects in which the union has considerable expertise: organizing workers and mobilizing voters.
He said that door-to-door canvassing for issues like a $15 minimum wage, union access and universal health care in the coming months would give way to electoral canvassing as next year’s voting approaches. Even then, issues will remain at the forefront, he said: the union and the Fight for 15 will continue to emphasize their agenda and only tout candidates who strongly support it.
That strategy is in contrast to the union’s approach in the last presidential campaign, when it endorsed Hillary Clinton during the Democratic primary in November 2015, even though Senator Bernie Sanders of Vermont appeared more in sync with its priorities.
“You learn every time you do stuff about how to do it differently, better,” Mr. Courtney said.
The key to the union’s program for 2018, Mr. Courtney added, will be to expand the universe of voters who turn out on Election Day.
The S.E.I.U. conducted a pilot project during the 2016 campaign in which it canvassed groups of voters largely in two heavily African-American wards of Detroit using a small-scale version of the campaign it plans for the year ahead.
Over all, about 62 percent of voters the union talked to during the pilot project cast ballots in the presidential election, versus turnout of about 38 percent of voters who it did not talk to, according to data provided by the union.
Applying the same percentage to all of Detroit’s voters would have produced about 40,000 more total votes in 2016, an amount that would have almost certainly secured the state for Mrs. Clinton.
Two people briefed on the initiative said the overall budget would approach $100 million, though that figure includes a substantial portion of money that the union was already likely to spend in these states. Even so, the initiative ensures that the political spending will primarily support outreach to voters on key issues, which would not necessarily have been the case.
By comparison, the union spent about $70 million on all federal political activity in 2012 and again in 2016, though the sums rose considerably once spending on local candidates and other nonfederal costs was included. Beyond the Midwest, the initiative will be aimed at a small number of competitive states elsewhere in the country, like Colorado, Florida and Nevada.
But the effort comes after the union indicated last year that it was reducing its budget by 30 percent, and some labor officials may question whether a focus on politics, whatever its short-term benefits, will divert the union from solving its more profound challenges.
In an essay published shortly after the presidential election, David Rolf, an international vice president of S.E.I.U. from Washington state, derisively predicted that one likely consequence of the Republican victories was that members would be asked to finance new forays into electoral politics, at a potential cost of seeding the experiments that could one day produce a viable successor to today’s unions. For example, some cities have funded worker organizations that help enforce local employment laws by identifying violations.
It was “hallucinatory,” Mr. Rolf wrote, to believe that “the hundreds of millions of dollars the labor movement spent just on Wisconsin gubernatorial elections between 2010 and 2014 couldn’t have been better used to build new forms of worker power.”
Because of an editing error, an earlier version of this article misidentified one traditionally Democratic state carried by President Trump in November. He won Michigan — not Illinois — along with Pennsylvania and Wisconsin.