WASHINGTON — Sen. Frank Lautenberg, the nation’s oldest senator at 89, has been sidelined in New Jersey with pain and weakness in his legs.

Lautenberg spokesman Caley Gray said Wednesday that the senator’s doctor has advised him not to travel. The Democrat has missed several votes in the Senate this week and last.

Gray said the senator was working to rebuild strength in his legs.

Lautenberg was sick with the flu in late December and early January. He missed several votes related to a $60 billion relief package for Superstorm Sandy.

Lautenberg announced last month he would retire next year and not seek re-election.

He was diagnosed in February 2010 with B-cell lymphoma of the stomach and underwent chemotherapy treatments until he was declared in June 2010 to be free of cancer.