(FORTUNE Magazine) – It's getting harder for boards to handpick new members this proxy season. More than 120 companies, including Motorola, Dell, and Time Warner, have recently signed on to new stricter corporate-governance rules that require a majority of shareholders to approve new board members. In the past the required "plurality" could be as little as one yea vote, rendering ballots virtually meaningless in an uncontested election. "This gives shareholders a credible threat," says Beth Young of the Corporate Library. Expect more companies to adopt the change this month; it's already on over 140 ballots.