WASHINGTON, April 18 (Reuters) - U.S. Senator Robert Menendez, the top Democrat on the Foreign Relations Committee, said on Wednesday he would oppose President Donald Trump's nomination of CIA Director Mike Pompeo to be secretary of state, in a blow to the nomination.

Menendez announced his decision during an address at a Washington think tank.

Pompeo can be confirmed by the full Senate without the committee's support. But that would make him the first secretary of state nominee at least since such votes were made public in 1925 not to win the panel's backing, weakening him as he goes into the job even if he is confirmed. Even his backers say they expect he would at best be confirmed by a narrow margin. (Reporting by Patricia Zengerle and Doina Chiacu; Editing by Chizu Nomiyama)