With the Supreme Court striking down laws that require government workers to pay union fees, one thing is clear: Most public-sector unions in more than 20 states with such laws are going to get smaller and poorer in the coming years.

Though it is difficult to predict with precision, experts and union officials say they could lose 10 percent to one-third of their members, or more, in the states affected, as conservative groups seek to persuade workers to drop out.

The court’s decision is the latest evidence that moves to weaken unions are exacting a major toll. Beyond the dropout campaigns aimed at members, conservatives are bringing lawsuits to retroactively recover fees collected by unions from nonmembers.

In May, President Trump signed three executive orders making it easier to fire government workers and reining in the role of unions representing federal workers.

Dropping out of a union is a more attractive proposition now that workers no longer have to pay a so-called agency fee, typically about 80 percent of union dues, if they choose not to belong to a union. (Those doing so generally account for a small fraction of the workers whom public-sector unions represent.)

In the five years after Michigan passed a law ending mandatory union fees in 2012, the number of active members of the Michigan Education Association dropped by about 25 percent, according to government filings, a much faster attrition rate than before. Its annual receipts fell by more than 10 percent, adjusting for inflation.

Still, the more interesting question is whether the unions, whatever the blow to their ranks and finances, will be substantially weaker.

Union leaders insist that they won’t — that the crisis posed by the case, Janus v. American Federation of State, County and Municipal Employees, has brought more cohesion and energy to their ranks.

“No one wanted this case,” said Randi Weingarten, president of the American Federation of Teachers. “But the gestalt around the country has been to turn an existential threat into an opportunity to engage with our members like never before.”

There are reasons to believe that the claim is not merely desperate bravado.

One parallel to the current development is a 2014 Supreme Court ruling known as Harris v. Quinn, which struck down mandatory union fees for home-based workers who serve private individuals but are paid through government programs like Medicaid.

As of late 2013, the Service Employees International Union represented about 60,000 public-sector home care and child-care workers in Illinois, about 40 percent of whom were union members. (The rest paid agency fees.)

Receipts for the service employees union local representing home-based workers in Illinois dropped significantly in the four years after the decision. But an aggressive membership campaign largely offset the loss of members.

It also built and reinforced personal relationships with members, who could be summoned to make demands of politicians in nearly every legislative district.

“Our members go and meet Sam McCann,” said Keith Kelleher, who until last year was president of the local representing these home-based workers, referring to a Republican state senator. “He says yes most of time because he’s got hundreds of members in his district.”

Public home-based workers in Illinois, a state with a notably anti-union Republican governor, continue to notch victories as a result. Last summer, home care workers won a 48-cent-an-hour wage increase from the state, up from an average wage of $13, in a budget that the legislature passed by overriding the governor’s veto. This spring, home child-care workers won more than a 4 percent raise.

In anticipation of the Janus ruling, major public-sector unions have invested heavily in recent years in reaching out to current members — an effort known as internal organizing — and to prospective members to keep their numbers from dropping precipitously and to create a more activist culture. They plan to continue funding these initiatives even if it requires cutting spending elsewhere.

Mary Kay Henry, the international president of the service employees union, said the union used projections derived from its experience after the Harris decision to cut its budget by 30 percent shortly after Mr. Trump was elected. She said the union, which represents about two million workers, roughly half of them in the public sector, was focusing its spending on recruiting members and mobilizing workers to face down employers and elect pro-labor politicians.

“We intend to prioritize the political and organizing work,” she said.

Government filings show that the union has cut contributions to organizations that it had traditionally supported, including the Children’s Defense Fund, People for the American Way, and the National Immigration Law Center. (The union says it provides nonmonetary support to some of these groups.)

At the same time, the union is investing tens of millions of dollars in a door-to-door canvassing initiative for the midterm elections, intended to turn out people who don’t normally vote.

Lee Saunders, president of the American Federation of State, County and Municipal Employees, said that his union’s two highest priorities going forward would be its internal outreach and helping to organize nonunionized workplaces, and that the union would probably “have to make adjustments” to fund these programs. The union spent more than $15 million during the 2016 campaign cycle supporting political candidates, parties and committees.

Mr. Saunders said the union, which represents over 1.2 million workers, had held one-on-one conversations with nearly 900,000 members since 2013. Among the goals of these conversations, he said, is to inoculate members against campaigns by conservative groups to urge them to quit.

“If someone knocks on their door talking about how you can get out of the union — ‘it would be so easy, you don’t have to pay union dues’ — our folks are prepared to tell them to get the hell off their doorstep,” he said.

Alexander Hertel-Fernandez, a political scientist at Columbia University who studies corporate and conservative efforts to weaken labor, said organized interest groups had traditionally had the greatest impact on elections by educating members about candidates and through on-the-ground canvassing rather than large campaign contributions. “It’s doubly so for unions,” he said, adding that the focus “seems like a wise decision, but the effectiveness has to be weighed against what happens to membership and overall revenues.”

The unions enjoy certain advantages. States like California and New Jersey have tried to ease the blow from Janus pre-emptively by passing legislation that, for example, guarantees public-sector unions access to new hires and their personal contact information to help in recruiting.

There is also a substantial wind at their back: a rising energy on the left during the Trump era. Workers in particular appear more willing to take to the streets and state capitols, including tens of thousands of teachers who walked off their jobs this year in conservative states to protest the underfunding of public education.

When the Supreme Court ruled last month that employment contracts could prohibit workers from bringing class-action lawsuits, activists in states like New York, Vermont and Oregon escalated their efforts to pass so-called private attorneys general legislation, allowing workers to bring cases on the state’s behalf that could benefit all affected workers, the same way litigation by an attorney general would.

“We’ve had many, many folks calling: ‘I heard about this legislation you helped design. How do we make this happen?’” said Deborah Axt, co-executive director of Make the Road New York, an advocacy group pushing the measure. Ms. Axt said the group planned to campaign for the legislation’s enactment this summer.

That kind of energy appears to be benefiting unions. A Gallup poll last summer showed labor’s approval at its highest level since 2003, and unions in West Virginia and several other states where teachers walked off the job this year report gains in members.

“We’ve seen a 13 percent jump in membership because of the walkout,” said Ed Allen, president of the Oklahoma City American Federation of Teachers. “We have over 300 people signed up to work in political campaigns. We’ve never seen those kinds of numbers before.”