Besides Senator Gruening, Senator Morse was the only other vote against the Tonkin Gulf Resolution. While Gruening disagreed because he didn't want to send Americans soldiers to die; Morse was focused more on whether or not the resolution followed the Constitution.

Bottom line: he thought Tonkin Gulf Resolution was unconstitutional.

The Constitution of the United States says pretty clearly that only Congress can declare war. Morse's issue was that the Tonkin Gulf Resolution was essentially giving the President the power to wage war, without a formal declaration.

He said during the Senate debate:

"I believe that history will record that we have made a great mistake in subverting and circumventing the Constitution of the United States." (Source)

His meaning? By not formally declaring war we are opening up the possibility of letting the President send military forces around the globe at his whim.

Morse channels his inner prophet and goes on to say:

"I believe that within the next century, future generations will look with dismay and great disappointment upon a Congress which is now about to make such a historic mistake." (Source)

Was he right?

Well, yes and no. Most people today look back at Vietnam and say that it was a disaster for the United States. However, there wasn't necessarily a huge backlash against Congress for the Tonkin Gulf Resolution specifically. It was more of a general anger that America was involved in the war at all, and if anyone took the blame, it was the President.

After all, Congress is five hundred or so faces—the President is just one.