BOSTON (Reuters) - New York City Comptroller Scott Stringer is wielding more clout as he presses more than 150 U.S. companies to detail plans to diversify their boardrooms.

Scott Stringer speaks during a primary debate for New York City comptroller in the WCBS-TV studios in New York, August 22, 2013. REUTERS/Frank Franklin II/POOL

Stringer oversees $181 billion in city pension fund assets. In a statement emailed by his staff to Reuters this week, he said he had gained leverage in the diversity campaign from a three-year-old push for companies to adopt so-called proxy access rules that give outside investors more power to nominate corporate board directors.

At companies now following those rules, or where they have been supported by investors, Stringer is now seeking more disclosures about the race and gender of current directors and their skill mix, the statement said.

Boards to date have not cast a wide net when seeking new members, Stringer said in the statement.

Stringer's office said companies it planned to press on boardroom diversity included Alliance Data Systems Corp ADS.N which lists one woman among its 10 directors, and health insurer Humana Inc HUM.N, with one woman among 11 directors.

An Alliance Data spokeswoman said the company is looking forward to hearing more from Stringer about his efforts and noted the company has scored well by other measures such as having women in leadership positions.

“Alliance Data’s board of directors routinely discusses board diversity and refreshment, and it recognizes the importance of this issue,” she said, via email.

Humana representatives did not respond to requests for comment on Thursday afternoon.

Highlighting the value of the so-called proxy access rules, Stringer said he also plans talks about the makeup of company boards.

“Now that we have that power, it’s time to raise our voice and demand change,” Stringer said the statement, adding that he was also looking for boards to bring in directors with expertise on topics like climate change.

Making U.S. corporate boards more diverse has become a hot topic in recent years, with big asset managers including BlackRock Inc BLK.N and State Street Corp STT.N pushing corporations to add more women and minority directors as a way to improve performance and decision-making.

Among the 527 largest public companies in the U.S., 20 percent of directors were women in 2016, down from 21 percent in 2014, according to a report by executive search and board advisory firm Egon Zehnder.

Tracking a board’s exact demographics can be complicated. A person’s race or ethnicity may not be obvious from a photo in an annual report, for instance. Stringer also plans to ask boards to note the sexual orientation of directors, as a voluntary disclosure.

Last year former U.S. Securities and Exchange Commission Chair Mary Jo White said its staff was preparing requirements that companies disclose more about their boardroom diversity, but few expect the agency’s new chair, Jay Clayton, to put the requirements into practice.

An SEC spokeswoman declined to comment.