Can Reagan beat Carter's record in the polls?

''The doubters have fallen off the wagon,'' says John Iseminger, a Hudson, Iowa, farmer.

Mr. Iseminger is talking about recent declines in Ronald Reagan's standing with the public. Looking at the President's support with his practical farmer's eye, he adds: ''The solid ones are still with him. Most of the farmers know it'll take time. He's trying hard. Reagan may have more support in another year than he does now.''

The Gallup, Roper, Yankelovich, Los Angeles Times, CBS, and ABC polls record the decline in Reagan's support, as doubts surface about federal deficits, recession, war in Central America, nuclear brinkmanship with the Soviets.

In Iowa, Mr. Reagan's approval rating has plummeted from a 62 percent approval rate last fall (26 percent disapproved) to almost even (42 percent approve, 43 percent disapprove) in early March. By 3 to 1, Iowa farmers say Reagan policies are hurting the farm economy, according to a Des Moines Register survey.

Reagan's approval rating has declined from a high - following the March 30, 1981, attempt on his life - much as Jimmy Carter's did over his first year in the White House. Both presidents were in the mid-60 percent range their first April, in job performance approval. By the second April, both were in the mid-40 s, according to the CBS News/New York Times surveys.

Despite such a remarkable similarity so far in the pattern of support for two very dissimilar presidents, analysts say Reagan can conceivably avoid the depths to which Carter support plunged by his third April.

The Republican President in a televised press conference Wednesday evening, and in a planned series of Saturday broadcasts, will try to reach past the Washington press corps and critical elites to the public itself.

Reagan still has the powers of incumbency. An anticipated positive gesture to the Soviets on arms control, a more conciliatory stance in coming weeks to avoid a budget impasse, his June travels to see European heads of church and state--such initiatives give Reagan opportunities to recast his image.

Also on the plus side, the public does not yet seem to blame Reagan for US economic problems. In Iowa, Reagan is still rated ''an effective leader'' by a margin of 2 to 1. Nationally, the public's ''overall impression'' of Reagan was similarly 2-to-1 positive, in the latest Los Angeles Times survey, taken March 14 to 17.

The public clearly likes Reagan the man more than his policies. On foreign and military issues, three-fourths of the public opposes sending military or other aid to El Salvador, regardless of who wins the election there, the Time-Yankelovich poll shows. Another policy, selling arms to Arab nations, is opposed by three-fifths of the public. More than two-thirds of the public wants to negotiate on disarmament rather than expand the US nuclear arsenal.

On the domestic side, the public would very much like to balance the federal budget--an ambition demoted by Reagan from a campaign promise for 1982 or '83 to a ''goal'' for 1985 or later. The public is less insistent on the military spending and tax cuts that Reagan wants.

How much the public is responding in its Reagan rating to the long grind of the recession, and how much it reflects the historical pattern of presidential reappraisal, is not clear. But if the historical forces are dominant, a reprieve lies ahead for Reagan before his fourth April in office.

''Almost any president will continue going down until January or February of his fourth year,'' says Burns W. Roper, president of the Roper Organization. ''Then he will start back up.''

In the near term, the public's discontent with Washington leadership may fall most heavily on Republican candidates for the US House in the November elections. Pollster George Gallup says Democrats could double their 51-seat House majority in November to 100 seats.

Even so, this would not necessarily mean Reagan's solid supporters had jumped off his wagon. It would certainly indicate that talk of a Republican resurgence was premature.