Melissa Gilbert (search), who has presided over a sometimes bitterly divided Screen Actors Guild (search) during the past four years, announced Thursday she won't seek a third term as president.

The former child star of "Little House on the Prairie" (search) acknowledged she would be leaving the presidency of SAG, the nation's largest actors union, with some degree of frustration.

"It is no big secret that there are problems within the leadership of SAG. There are rifts that may very well be irreparable," Gilbert, 41, said in a statement.

"All of that aside, I still believe the Screen Actor's Guild was, is and will always be the most powerful performers union in the world, and I want the future leaders of my beloved union to know that I will be watching them — like a hawk," the statement read.

During her two terms, Gilbert pursued an aggressive reform agenda that was only partly successful.

One of her most ambitious proposals, to merge SAG with the American Federation of Television and Radio Artists union, suffered a bitter defeat two years ago. Although nearly 58 percent of SAG members voted approved the merger, the number was just short of the required 60 percent.

She also tackled several other thorny issues, including propping up SAG's troubled health and pension fund and consolidating field offices to cut costs.

During her tenure the union won new contracts for actors who work in commercials and, separately, for those who work in television and movies. Earlier this month it reached tentative agreement on a new contract for actors who supply the voices for video games.

"The past four years have been remarkable, at times frustrating, but incredibly rewarding," she said.