WASHINGTON — Thousands of public pools in cities and towns nationwide will be taking a fresh look at their safety systems after federal regulators changed course Wednesday on measures required to keep swimmers, especially children, from getting trapped in pool drains.

The Consumer Product Safety Commission voted 3-2 to revoke guidance it provided pool operators nearly 18 months ago about how to comply with a sweeping 2008 law aimed at preventing drain suction from trapping swimmers under water. The law was passed in response to several horrific child entrapment deaths, including the 7-year-old granddaughter of former Secretary of State James Baker.

The new guidance requires public pools with a single main drain to have a back-up system that could shut the suction of the drain. Previously, they had been told they could use a new “unblockable” drain cover, usually a dome-shaped piece of equipment that covers the drain and prevents someone from getting trapped. The concern is that the “unblockable” drain cover could break, come loose or be improperly installed.

The move means public pools with those single-drain systems would have to buy new and costly back-up systems. Some pools may close if they don’t have the new equipment by next May.

The new guidance comes after one of the five commissioners at the agency decided to change his earlier vote on how pools should interpret the law.

After siding last year with the two Republican commissioners, Democrat Bob Adler said Wednesday he has had a change of heart about what Congress intended when it wrote the law.

“My previous interpretation is wrong,” Adler said, explaining that he has spent months talking to lawmakers who helped write the law and to industry officials, as well as hearing from parents who lost children in entrapment accidents.

The decision followed nearly two hours of contentious debate among the commissioners at a public meeting.

The overall impact on cities and states is not clear. Neither the CPSC nor the industry could provide figures on how many of the nation’s estimated 300,000 public pools and spas have single-drain systems.

Bigger pools with multiple drains are not affected by the vote.