About 200 Florida citizens will come to the White House Friday for an "issues briefing" by President Carter and his top policy aides.

Next month, William (Bud) Dunfey, a leading New Hampshire business-man-politician, is expected to be named as a member of the U.S. delegation to the United Nations General Assembly, which will make make him the third Dunfey brother to receive recognition through a prestigious federal post.

When Lord Mountbatten of Burma was assassinated, it occurred to someone at the White House that Chicago's Mayor Jane Byrne would be an excellent person to accompany Averell Harriman to the state funeral. And the same person was pleased to provide the president as the main speaker for the mayor's October fundraiser.

Similarly, those putting together the guest list for the president's reception for Pope John Paul II remembered that in 1976, Carter's first win was helped by the Catholic voters in Iowa, and put "a number" of Iowa names on the list.

That Florida, Illinois, Iowa and New Hampshire are among the earliest states choosing delegates to the 1980 Democratic convention is what politicians call a happy coincidence.

After two years in which Democratic Party politicians regularly accused him of ignoring the political perquisites of office, Jimmy Carter appears to have discovered the advantages of incumbency.

And as the likelihood grows that he may be challenged for renomination by Sen. Edward M. Kennedy (D-Mass.), Carter's supporters are virtually advertising his readiness to reward his friends in tangible and intangible ways.

In New Hampshire, the site of the first primary, for example, a $47,500-a-year, part-time job as a member of the U.S.-Canadian International Joint Commission has been given to Jean Hennessey, a member of the Democratic National Committee and a political ally of Gov. Hugh Gallen (D), a staunch Carter supporter.

The same job once was discussed with Dudley W. Dudley, who is leading the Kennedy write-in campaign in the state.

Another Gallen lieutenant, Mary Louise Hanock, resigned from the state Senate in July to become a $32,442-a-year special assistant to the regional administrator of the Department of Housing and Urban Development, who is a former Kennedy aide.

A White House official denied that there are orders out to pick off as many past and prospective Kennedy supporters as possible before the senator makes his decision on challenging Carter.

But the overtures to such longtime Kennedy allies as Mayor Byrne and the Dunfey brothers have not gone unnoticed in the political community.

What Carter is doing is hardly novel. President Ford used the same technique when he was being challenged for nomination by Ronald Reagan in 1976, and so have other presidents when their political futures seemed to be in jeopardy.

But it represents a considerable turnabout for a man who criticized Ford for using the White House to help his campaign and reelection and who leaned over so far backward himself to avoid such political gestures that many Democrats felt they were being snubbed in the first two years of his presidency.

For the record, Tim Kraft, the manager of Carter's reelection campaign, said yesterday that "incumbency is a mixed blessing." But then he laughed and added, "If it's a handicap, I'm glad we work under it."

As Ford learned, it is not a guarantee of election. Public doubts and dissatisfactions with the record of the man in office, as reflected in Carter's standing in recent polls, ultimately can doom his efforts to stay in office.

But along the way, the White House conveys enough advantages to assure that a president determined to exploit them will not be an easy man for anyone to dislodge.

The perquisites of office include patronage appointments, the delivery of discretionary federal aid and the prestige of White House invitations, plus the unrivaled communication, transportation and support facilities available to the president.

Today, for example, Carter will travel in Air Force One to heavily publicized "nonpolitical" events in Connecticut and Ohio (two more important battleground states with strong Kennedy factions), while his wife, Rosalynn, is in New Hampshire and Vice President Mondale is meeting in Washington with groups ranging from Jewish leaders to shopping center managers.

But the direct efforts of the principals, like Mondale and the Carters, are only the tip of the iceberg of incumbency influence in the kind of battle now beginning. A major focus of the White House strategy has been the familiar "endorsement" game, getting public officials, party leaders and spokesmen for important interest groups committed early to the president.

With some conspicuous prodding from the presidential aides who serve as their liaison men on all problems with the federal government, 20 Democratic governors gave their endorsement to Carter at a caucus last July.

It was a one-day headline across the nation, but in New Hampshire, the support of Gallen, who was one of the 20, has become a linchpin of the Carter campaign.

In public speeches and private meetings, Gallen has remined Democrats that the White House came to his aid with fund raising and campaign help when he was waging an underdog struggle against incumbent Republic Meldrim R. Thomson in 1978. He has met with Carter three times this year to assure that New Hampshire will have an adequate supply of heating oil for the winter, Gellen is telling the voters.

"The president instructed the Energy Department to see that the supplies were in hand, and he has kept on top of them to see that we will have what we need," Gallen says.

The implication is that Kennedy may be a fine man, but only Carter can deliver the fuel oil.

When Gallen sent a letter to New Hampshire Democrats inviting them to join him on the Carter campaign steering committee, more than 700 of them accepted and sent in more than enough money to pay for the mailing.

In the busy Carter headquarters two blocks from the state capital, Chris Brown who managed the Carter victory in the 1976 primary, marveled at the change in circumstances.

"We won without any endorsements back then," he said. "I'm not used to the amount of support we have this year."