If you report politics, it helps a lot to fix clearly in your mind what the voters are thinking and saying before you are inundated in the rhetoric of the candidates. For that reason, I have begun every presidential campaign year since 1968 by doing several weeks of door-knocking on my own.

This year, I concentrated on two sets of precincts in the Dallas and Detroit metropolitan areas picked by elections expert Richard Scammon as being typical of their states.

These precincts are key battlegrounds. If either Jimmy Carter or Ronald Reagan carries both Michigan and Texas, he is virtually assured of winning the election. If the two states split, as they did four years ago, we are almost certain to have another cliffhanger.

Talking to voters in their homes this way, you don't reach enough people for your findings to have any statistical validity. But you sure get into your mind the hopes and fears that form the mental lens through which the American people view this election.

On a subjective basis, I come away with the feeling that Jimmy Carter has an uphill climb to reelection -- maybe a tougher race than the national polls now indicate.

Carter has closed ground in the past month. His main challenger, Reagan, has made some slips the voters have noted disapprovingly. But, as in the primaries, those verbal graffes do not seem to cause a continuing erosion of support. Voters seem forgiving of Reagan, in part because there is an almost universal impression -- so far unchallenged by the Democrats -- that he was a competent, effective governor of California.

John Anderson, the Independent, is only marginally in the voters' consciousness. Most of them know someone of that name is running, but they don't know much about him or what he stands for. The televised debates starting next Sunday will fill in some of that missing picture and very likely increase Anderson's support, at least for a while.

The peril to Anderson is that even those who find him an attractive option are skeptical he can win. And the word "spoiler" comes up much too often to be comfortable for Anderson's prospects.

But for now, the voters' decisions seem to revolve around their judgment of Jimmy Carter and his record -- and that is why he is in trouble.

If you find a self-described Democrat over 45, a black, a Hispanic, a school-teacher or another government employee at home, you have probably met somebody who is sticking with Carter. So are a number of women who tell you that, whatever his other failings, this president for four years has avoided drafting their sons or sending them into war. The "peace" issue may grow as the campaign develops.

But voters under 45 -- particularly men -- are turning away from Carter in droves, because of unemployment, inflation, the hostages in Iran and the pervasive sense they expressed that America has been diminished -- not enhanced -- in the eyes of the world during his presidency.

A Dallas policeman in his early 40s said something striking. "To me," he said, "Reagan is another John F. Kennedy. He just belongs to a different party."

"How is that?" I asked.

"Reagan's motto is 'Let's make America great again,'" he said. "To me, that's what John F. Kennedy meant when he told people to ask what they could do for their country, not just ask what their country could do for them. Kennedy took the responsibility for being a leader. Carter, he just tells you it's everybody else's fault things are getting so bad."

If this election were simply a referendum on Carter's first term, the feeling you get in these precincts is that the president would lose and lose badly. But there are misgivings -- some serious -- about the alternatives, and that is what makes the outcome uncertain.

What you sense over and over again is the feeling that the challenges facing America seem to drawf the dimensions of the men who aspire to its leadership. The voters I met do not think they have been given a choice of three strong men of unblemished reputation and superb qualifications. Quite the contrary. A reporter is asked insistently: "How come these are the only alternatives?

The irony is that these really are popular choices. Carter and Reagan won with more primary election votes than any previous pair of majority-party nominees ever accumulated. John Anderson is on the ballot by petition of hundreds of thousands of other citizens.

That ought to be a source of comfort to the voters, as they face their choice. But it self-evidently is not.