The nation’s consumer watchdog is adopting a rule on Monday that would pry open the courtroom doors for millions of Americans, restoring their right to bring class-action lawsuits against financial firms.

Under the Consumer Financial Protection Bureau rule, banks and credit card companies could no longer force customers into arbitration and block them from banding together to file a class-action suit.

The change would deal a serious blow to Wall Street and could wind up costing financial firms billions of dollars.

More immediately, its adoption is almost certain to set off a political firestorm in Washington, where both the Trump administration and House Republicans have pushed to rein in the consumer finance agency as part of a broader effort to lighten regulation on the financial industry.

Under the Congressional Review Act — a 1996 law that had been rarely used before the current Congress employed it to reverse 14 rules from the Obama administration — lawmakers have 60 legislative days to overturn the rule blocking mandatory arbitrations. The rule could take effect next year.

The Chamber of Commerce and other pro-business groups have belittled the rule as nothing more than a gift to class-action lawyers, who tend to be Democratic donors.

But as much as Republicans deplore the consumer protection agency, they may find it difficult to kill a rule that could have wide populist appeal. Across the country, judges, prosecutors and regulators have decried arbitration clauses for allowing corporations to circumvent the courts and for taking away the only tools citizens have to fight illegal or deceitful business practices.

The rule is one of the signature efforts of the Consumer Financial Protection Bureau, which was created in 2010 as part of the Dodd-Frank regulatory overhaul to safeguard the rights of millions of Americans in the aftermath of the mortgage crisis.

At a time when Dodd-Frank has come under attack, the arbitration initiative from the consumer finance agency — which operates independently from the Trump administration — is a provocative stand against the prevailing political tide in Washington.

Indeed, the rule is largely unchanged from when it was issued in draft form in May 2016 and the agency began soliciting comments from industry.

It is that kind of independence that has drawn particular ire from Republicans.

Last month, the Treasury Department issued a report recommending that the Consumer Financial Protection Bureau be neutered, accusing it of regulatory overreach and calling for the president to be able to remove its director, Richard Cordray.

Supporters of the agency say arbitration is exactly the kind of issue that requires independence from corporate interests.

The rule will unwind a series of brazen legal maneuvers undertaken by major American companies to block customers from going to court to fight potentially harmful business practices.

“These clauses allow companies to avoid accountability by blocking group lawsuits and forcing people to go it alone or give up,” Mr. Cordray said in a statement.

Over decades, financial institutions, led by credit card companies, figured out a way to use the fine print of their contracts to force consumers into private arbitration, a secretive process where borrowers have to go up on their own against powerful companies with deep pockets.

Prevented from banding together in a class and pooling their resources, most people simply abandon their claims entirely, never making it to arbitration at all.

The new rules could change all that when it comes to consumer finance. While the protections would not apply to existing accounts, consumer could pay off old loans and get new accounts that would fall under the new rules.

The new rules do not explicitly outlaw arbitration, but industry lawyers say that they will effectively kill the practice.

“If this rule goes into effect, what we are going to see is a huge avalanche of litigation and a loss to consumers of the benefits of arbitration,” said Alan S. Kaplinsky, a lawyer with the firm Ballard Spahr in Philadelphia, who is widely considered the father of arbitration clauses.

To Mr. Kaplinsky, who opposes the rule, arbitration offers a faster and more efficient way to resolve legal disputes.

In the debate about arbitration, those assertions were almost entirely anecdotal. There is no federal database that tracks arbitrations and the process is entirely secretive.

To get beyond the anecdotal, The New York Times assembled its own database of arbitrations in a series of articles in 2015 that showed few people ever go to arbitration.

In financial disputes, the numbers are particularly startling. In its investigation, The Times found that between 2010 and 2014, only 505 consumers — a fraction of the tens of millions of Americans whose financial contracts have arbitration clauses — went to arbitration over disputes of $2,500 or less.

That reluctance is why one federal judge remarked in an opinion that “only a lunatic or a fanatic sues for $30.”

By banning class actions, companies essentially squashed challenges to practices that ranged from predatory lending and wage theft to sexual discrimination and medical malpractice.

Among the class actions derailed over the years by arbitration was a case brought by Citigroup customers who had accused the bank of tricking them into insurance that they were never eligible to use. In another, a group of merchants challenged American Express over high processing fees.

The rule from the Consumer Financial Protection Bureau would apply only to the financial companies regulated by the agency and would not touch arbitration clauses buried in the fine print of nursing homes or employment contracts.

Clauses embedded in those contracts have pushed disputes about elder abuse, sexual harassment and even wrongful death out of view.

Recognizing that problem, the federal agency that controls more than $1 trillion in Medicare and Medicaid funding proposed a rule last September that would have barred any nursing home that gets federal funding from requiring residents resolve disputes in arbitration. But the protection was fleeting. Soon after the Mr. Trump took office, the administration moved to scrap the rule.

In some ways, the fate of the nursing home rule adds urgency to the efforts by the consumer bureau. The agency’s rule represents the first significant blow to arbitration since a pair of Supreme Court decisions in 2011 and 2013 enshrined its use.

Those decisions, which initially drew scant attention outside the cloistered legal world, upended decades of jurisprudence that had been put in place to protect workers and consumers.

To stop the spread of class-action lawsuits, a coalition of credit card companies used an arcane federal law dating to 1925 that formalized arbitration as a way for companies of equal bargaining power to resolve corporate disputes. Starting in the early 2000s, the credit card companies began to use arbitration for disputes with their customers.

Today, it is virtually impossible to apply for a credit card, rent a car, get cable or internet service, or shop online without agreeing to private arbitration.

As arbitration crept into tens of millions of contracts, prosecutors, judges and lawmakers starting sounding alarms. In the Dodd-Frank law, the agency was specifically mandated to examine arbitration.

The analysis culminated in a 728-page report, released in March 2015, that showed how few consumers went to arbitration once they were prevented from joining a class action. For those who did go through with arbitration, the agency found, the results were dismal. During the period studied by the agency, only 78 arbitration claims resulted in judgments in favor of consumers, who got $400,000 in total relief.

The financial industry balked at the findings, arguing that on a person by person basis, consumers wind up with more money in arbitration than in class actions.

But law professors and judges, including some appointed by conservative presidents, say that the total amount of money obtained in class actions is beside the point. Class actions, they argue, are intended to help big groups of people get back small amounts of money — say a $35 overdraft fee. More important, they argue, class actions can push companies to get rid of questionable business practices.

Big banks, for example, had to pay more than $1 billion to settle class-action lawsuits, which started in 2009, that accused them of monkeying with checking account policies to maximize the number of overdraft fees they could charge customers.

In the aftermath of the litigation, seven of the banks involved have adopted arbitration clauses to their contracts.