As the board of the Corporation for Public Broadcasting prepared to announce his successor as chairman, Kenneth Y. Tomlinson defended his tenure on Thursday, saying he had no regrets in trying "aggressively" to balance what he said was overly liberal programming in public television and radio. Board members, who plan to meet Monday in Washington, said this week that the leading candidate to become the corporation's next chairwoman was Cheryl F. Halpern, a Republican fund-raiser and former chairwoman of the Republican Jewish Coalition. SNIP Mr. Tomlinson has come under heavy criticism from Senate Democrats, who say he has injected politics...