Why was Senator Wayne Morse against the Tonkin Gulf Resolution?

Senator Wayne Morse (Democrat of Oregon) was one of only
two Senators to oppose the Gulf of Tonkin Resolution.  He opposed it partly because he
opposed the war in general, but he also opposed it for constitutional
reasons.

Basically, Morse believed that the President could
not take actions such as the Resolution would have allowed him to do unless the Congress
had already declared war.  Morse was saying that, unless Congress declared war, the
President could not do the things the Resolution would have allowed him to
do.

This part of Morse's opposition, then, was based on the
idea of separation of powers.  He believed that this kind of massive involvement in a
war required an actual declaration of war by Congress.