President Reagan and the Republican National Committee yesterday gave the red-carpet treatment -- including breakfast at the Sheraton Grand on Capitol Hill, White House photo opportunities and a presidential speech -- to an impressed group of about 100 local officials who have abandoned the Democratic Party for the GOP.

"Let the other party have the entrenched interests and power brokers and special-interest politics; we don't have any of that. We just have the people," Reagan said to the GOP converts in the White House State Dining Room.

"I was moved. I was choked up when I met the president," said Jack Marshall, a former Democratic alderman in Tupelo, Miss., who last week became the city's GOP mayor. "I wish my mother could have been there."

The event is part of a drive by the Republican National Committee to capitalize on Reagan's landslide reelection and on polls showing increased voter identification with the Republican Party. The RNC has put $500,000 into Operation Open Door, aimed at converting 100,000 Democrats in four states -- Louisiana, Florida, Pennsylvania and North Carolina -- and at converting as many elected Democrats as possible.

The largest delegations of converts came from Texas, Louisiana and Mississippi -- Sun Belt states where the shift to the GOP was strikingly high last year, according to pollsters for both parties.

Since then, however, one of the few real tests of partisan strength after the 1984 election was held last week -- the municipal contests in Mississippi -- and the Democrats more than held their own.

In 20 cities with populations above 10,000, the number with Democratic mayors rose from 14 to 16 while the number of GOP mayors fell from five to three. One city remained governed by an independent. In Jackson, which changed its form of government, the city went from a Democratic mayor and two Republican city commissioners to a Democratic mayor and a 6-to-1 Democratic majority on the city council.

"We didn't do as well as we had hoped," said Ebbie Spivey, chairman of the Mississippi GOP. "If I were a Democrat, I would be boasting, too."

Keith Hill, the Mississippi Republican Party executive director, contended that the GOP suffered a public relations defeat in the municipal elections but made gains in many of the state's smaller communities. Overall, he said, the number of GOP elected municipal officers grew from 89 to 113.

In Brookhaven, population 10,800, the mayor and two aldermen who had switched from the Democratic Party to the GOP were defeated. In Brandon, population 9,626, three switched, including the mayor, and all won, most facing no Democratic opposition.

In his speech, Reagan was not as harshly critical of the Democratic Party as he was during a Memorial Day trip to Florida when he accused Democrats of "pitting white against black, women against men, young against old." Instead, he told the enthusiastic audience that the once-sedate GOP has been enlivened by leaders "lobbing around intellectual hand grenades . . . . And the supply-siders, the neo-cons, the New Right, the new Republican majority -- whatever you call them -- they took the Grand Old Party and made it the Grand New Party with great new power -- the GNP."

In a minor counterattack, New Hampshire Democrats said yesterday that Louis D'Allesandro, former executive councilor of New Hampshire, the second highest elected office in the state, was expected to switch from the GOP to the Democratic Party Wednesday. "In the rest of the country, they may be switching to the Republican Party, but in New Hampshire, they are coming over to the Democratic Party," said George Bruno, New Hampshire Democratic chairman.