Sen. Gordon J. Humphrey, R-N.H., who rose from political obscurity to become one of the nation's most outspoken conservatives, announced Monday he would not seek election to a third term in 1990.

"I'm here today to announce that two terms is plenty," Humphrey, 48, told a packed news conference in his Concord office. While Humphrey called the power of politics "addictive," he said another six years in the Senate "would be just too darned long."Humphrey's decision is expected to open the floodgates of candidates, Republican and Democrat, for the office. He is the second Republican senator to announce he is not seeking re-election. Sen. William Armstrong of Colorado announced he would not seek another term earlier this year.