President Carter yesterday called on the Democratic Party to unite over the summer and prepare for what he predicted will be a fall campaign of "demagoguery" and "simplistic solutions" by his likely Republican opponent, Ronald Reagan.

"The American people know that we can meet the problems much better than Ronald Reagan and the Republicans, who are looking back to the 1950s rather than forward to the 1980s," Carter told about 100 state party officials at the White House.

The party leaders, who were called to Washington yesterday to begin planning for the general election campaign, heard the president rehearse some of the main themes he is expected to use against Reagan.

Carter also used the occasion to appeal to Democrats to unite behind him, never mentioning the continuing challenge by Sen. Edward M. Kennedy (D-Mass.), who has said he will remain in the Democratic race until the party convention in New York in August.

"It is not easy in a time of change to provide leadership without unity in the party," Carter said.

In the next few weeks, the president said, Democrats will have an opportunity "to pull the party together, to unite in a common effort to face a formidable challenge" in the fall.

Repeating what he told Kennedy Thursday during their meeting at the White House, Carter said disputes within the party should be settled at the convention by the delegates selected "over a long, tedious, sometimes divisive primary season."

Carter, who won 51 percent of the total Democratic primary vote to Kennedy's 38 percent, has an overwhelming lead in convention delegates, and is now concentrating on attempting to avert a bitter convention clash between his and Kennedy's supporters that could weaken him in the fall.

The president's remarks about Reagan were some of the sharpest he has used. He predicted that in the fall the Republicans will offer the country a series of "quick fixes and a long list of simplistic solutions."

In his recent public appearances, Carter has been testing some likely themes for the fall campaigns and yesterday was no exception. He stressed, as he has been doing, the gravity of the outcome of the general election, for it will decide "who will be president, who will be responsible for peace or for war, who will be responsible for keeping people at work or for unemployment . . .

"These things, affecting the lives of people, will be the prevailing factor."

This is likely to be a main Carter theme in the fall as the president attempts to portray Reagan as an unknown, and therefore risky, alternative to his continued leadership.

White House press secretary Jody Powell said later that Carter was not attempting to suggest that a Reagan presidency would increase the risk of war, but only that "voters in a general election tend to treat their vote more seriously than they do in a primary."

Earlier, Democratic National Committee Chairman John White said the state chairman and other officials had been invited here to plot strategy on how to mobilize forces for the fall campaign.

The purpose of the session was to kick off a late-starting effort to aid the party's nominee. While each major party presidential nominee is limited by law to spending $29.4 million in public funds in the general election campaign, each national party can raise and spend an additional $4.6 million on behalf of its nominee. State and local parties can spend additional hundreds of thousands of dollars on registration, voter turnout and organizational activity.

The Republican National Committee has been raising money for this campaign for three years.

At a news conference, White dropped hints that it would be helpful if Kennedy conceded the Democratic nomination to the president.

"If we unify the party, I think we're a cinch at winning" in November, he said. "If we continue this, I think we can do to Jimmy Carter what we did to Hubert Humphrey."

In 1968, with the Democratic deeply split over the Vietnam war, Humphrey lost narrowly to Richard M. Nixon.