Reagan reported wedded to MX despite arms vow

President Reagan would ignore terms of the 1970s arms control treaties, despite his pledge to abide by them, if they interfered with his plan to deploy MX missiles, according to a high-ranking administration official.

Though the President agreed not to undercut the SALT treaties, he is aware that ''our principal purpose is to maintain a survivable deterrent,'' the official said, and he would not hesitate, if it became necessary, to take actions inconsistent with SALT.

The official drew a distinction between not undercutting in a legal sense and literal adherence to every point. He said that the MX plans would not technically violate the President's pledge.