If there's one man around this town who can be called an optimist it is Hubert Horatio Humphrey. This is all the more remarkable in view of the blows he has taken over the years of his experience in riding the political roller coaster. On top of it was a major cancer operation with a follow-up of chemotherapy, which he cheerfully describes as his bi-weekly dose of poison.

Being Lyndon Johnson's vice president was like being chained to the Loch Ness monster. Then came the 1968 election when, with Johnson glowering in his retreat in Texas, Humphrey failed until too late to disavow the Vietnam war. The victory by Richard Nixon was by such a squeak that with it went all the sad might-have-beens for Humphrey.

The presidential virus is all but incurable and in 1976 Humphrey toyed with the idea of running again. He had the good sense, however, to stay out of the primaries. Today, his past ambition is behind him, and it is no exaggeration to say that he is devoting himself with all the intensity of the Humphrey temperament to working for the success of his President and his country.

I had a talk with him the other day between long and arduous stints on the Senate floor. He spent three consecutive days shepherding the foreign-aid bill to passage, with sessions running until 9 p.m. or even later. Humphrey is one of four senators who have regular weekly meetings with Carter, along with five House leaders.

"On balance I would say he is doing all right," Humphrey said after speaking of certain differences between the Congress and the White House. "i am not trying to protect him. I was not on the bleachers for him, but every time I work with I feel a little better.

"I have been at a lot of meetings with President Kennedy, President Johnson, and a few other Presidents. But these are the most candid, open and frank discussions I have ever experienced. I think he does realize that the relationship got off ato a bad start. He has been told so in so many words by Bobby Byrd [Senate majority leader] Tip O'Neill [Speaker of the House], and the rest of us chirp in and let him know:

"'Mr. President, it is one thing to cancel our projects, but it is another how you do it. It is one thing to send messages up here, but it is another bow you follow up. And I suppose that if there is any weakness in the administration it is that they are trying to do too much too soon.'"

Humphrey contested with Byrd for the leadership post and came out second best. His first move was to go to Byrd to say, "Bob, I want you to be a great leader; I want to help you. I don't know how much time I've got around here but I want to help you." They have had a close and friendly partnership ever since.

The President has taken a long time, in Humphrey's view, to realize what a hold Congress's new budgeting procedure has over federal spending. The spending guidelines laid down by the Congressional Budgeting Office, under the direction of Sen. Edmund Muskie (D-Maine), are jealously guarded, taking precedence over the executive budget.

"So he is experiencing what I would call a very normal, healthy, competitive relationship between the executive and the Congress on the separarion of powers, considering that the Congress today doesn't roll over like it used to. It really is a different Congress since Vietnam and Watergate. It's very assertive."

And Humphrey thinks that is a good thing. He is not afraid to ask the tough questions. When he had Chairman Arthur Burns of the Federal Reserve System before the Joint Economic Committee, he "blew the whistle on him" with respect to the loans advanced by New York banks to developing nations. "And the banking fraternity up in New York went crazy." Burn's response to the banks was, "Get your house in order."

Humphrey gives him credit for that, although he believes the power the chairman wields is too great, particularly when the chairmanship is a holdover from a previous administration. Although he is for the independence of the Fed, he thinks the chairman and the President should have coinciding views on fiscal policy. "Otherwise you can reduce taxes here $50 billion and Arthur Burns can raise the discount rate one-half of one per cent and, hell, it is all gone."

Having renounced ambition, Humphrey is as free as a bird. IN the past his weakness was his trust of associates who were undeserving of that trust. A man of compassion and an excetpionally keen mind, his mistakes were those of the heart rather than the head. Now, as an independent agent, he is following his own course, and that course is winning him kudos as never before.