Republican presidential nominee Ronald Reagan today used rural Mississippi to launch a three-day swing that reflects the diversity and the difficulties of his approach to the campaign.

Reagan spoke here to a conservative rural audience assembled at the Neshoba County Fair. Then he flew to New York where he is scheduled to make what his aides call "a major, substantive speech" to the National Urban League on Tuesday.

The Neshoba County Fair has been a traditional forum for the outpourings of segregationists such as former Mississippi governor Ross Barnett. The Urband League is a 70-year-old multiracial organization which has long been in the forefront of promoting jobs, training and housing for blacks and other minorities.

While it doesn't show on his schedule, Reagan plans to drop in Monday at the New York hospital where Urban League president Vernon Jordan its recovering from gunshot wounds from a sniper attack June 5 in Fort Wayne, Ind.

President Carter will make the same pilgrimage Wednesday, speaking to the Urban League and visiting Jordan. Carter's Democratic rival, Sen. Edward M. Kennedy, and independent candidate John B. Anderson will address the Urban League Monday.

In his speech to a shirt-sleeved crowd at the Neshoba and County Fair, Reagan said he "believed would do everything he could to reorder priorities to "restore to states and local governments the power that properly belong to them."

Reagan also made mild criticism of Carter, who has described the Republican nominee as "irresponsible."

"I'll admit to that, if he'll admit that he's responsible," Reagan said.

Like most Republican presidential candidates, Reagan has found it difficult to attract support from black voters. In the view of some, he compounded the problem last month when he failed to respond to a speaking invitiation from the NAACP.

Reagan and his aides blamed the nonresponse on a scheduling foulup. Even so, Reagan had time on his schedule to address the convention and instead elected to vacation in Mexico.

Scheduling difficulties and internal disputes also have plagued the planning for this trip, Reagan's first campaign swing since he was nominated for president at the Republican National Convention in Detroit last month.

Originially, Reagan was scheduled to make the Urban League appearance first, and then fly to deliver his speech here at the Neshoba County Fair.

But some in the campaign objected to the symbolism of Reagan going to a community where three civil rights workers were slain with the complicity of local police officials in 1964.

"It would have been like we were coming to Mississippi and winking at the folks here, saying we didn't really mean to be talking to them Urban League folk," said one Reagan source. "It would have been the wrong signal."

The other side of the argument was made by those who see Mississippi, which Carter carried by only 14,000 votes in 1976, as the prime example of a Deep South state that is ripe to go Republican.

Rep. Trent Lott (R-Miss.), who met Reagan on his arival at Meridian, said that GOP presidential nominee's presence gave the candidate a good chance of carrying this state.

The Reagan campaign's hand had been forced to some degree by local announcement that he would go to this fair, a popular summer gathering place for rural Mississippians.

After his Urban League speech in New York, Reagan will fly to Chicago on Tuesday and meet with the editorial board of Johnson Publishing Co., publisher of the magazines Ebony and Jet.

More than most campaigns, the Reagan effort has been hampered by logistical problems. Schedules are slow to be prepared and changed frequently. As one standard quip on the candidate's plane goes, if Reagan is to succeed as a president, he would have to manage the country much better than he does his campaign schedule.

But the dispute over the current trip has been less a logistical one than a genuine intracampaign disagreement over what Reagan should be doing in this so-called pre-campaign period.

His strategy calls for him to focus on traditional Democratic groups -- the working-class voters centered in eastern and midwestern cities. While this does not foreclose the "southern strategy" used in the past by Barry Goldwater and Richard Nixon, it is supposed to relegate it to a secondary role.

However, Reagan is known to believe that he can wage an effective campaign in all sections of the country, appealing simultaneously to rural southerners and to urban northerners with long ties to the Democratic Party.

The varied schedule of this trip reflects that belief.