Commissioner Roel C. Campos will leave the Securities and Exchange Commission in one month and return to the private sector after five years of service, the SEC said Thursday.

The Democrat and former prosecutor is serving his second term, which was set to end June 5, 2010. His departure comes amid speculation that Annette Nazareth, the commission’s other Democrat, may leave by year’s end. The five-person commission cannot have more than three members from the president’s party, so any replacements — which have yet to be named — will have to be Democrats. SEC chairman Christopher Cox and the other commissioners, Kathleen Casey and Paul Atkins, are Republicans.

In its announcement noting Campos’ departure, the SEC commended him for his work in international affairs. He represented the commission when dealing with international regulators and serving as vice chair of the Technical Committee of the International Organization of Securities Commissions. “At a time when international collaboration among securities regulators has been key to our enforcement and investor protection missions, Roel Campos has led the way,” Cox said. “And his initiatives for ordinary investors — including his work to provide better disclosure for workers with 401(k) plans — have helped keep America’s markets the best in the world.”

During his tenure, Campos also pushed for better financial disclosures, improved corporate governance, greater enforcement capabilities for the SEC, and more input from shareholders in director elections. The latter issue remains unresolved at the SEC. He departs as the commission wrestles with the question of shareholder influence in the selection of directors and access to corporate proxies, which he favors.

In addition, Campos has advocated on behalf of small businesses. Last year he argued that smaller companies should have more time to comply with the costly internal-control provision of the Sarbanes-Oxley Act. Although Campos is a Democrat, he was frequently aligned with Republicans on the commission when it came to promoting regulations on hedge fund managers, stock trading, and mutual fund governance.

A graduate of Harvard Law School and the UCLA Graduate School of Business, Campos is also noteworthy for being the SEC’s first Hispanic commissioner. He created a diversity initiative at the agency to help recruit minority employees.

“I have been quite fortunate and privileged to have served the American investor at a crucial period in the history of U.S. markets,” said Campos.