Rep. Newt Gingrich (R-Ga.) and two other conservative Republicans yesterday blasted Maryland senatorial candidate Rep. Michael D. Barnes as a "radical" whose opposition to aid for Nicaraguan rebels inadvertently helps the Soviet Union.

In a news conference billed "Mike Barnes, McCarthyism and Nicaragua," Gingrich, Rep. Robert S. Walker (R-Pa.) and Rep. Duncan L. Hunter (R-Calif.) challenged Barnes to a series of debates, saying that Barnes is attempting to smear Reagan administration attempts to get $100 million in aid for the rebels in Nicaragua. They said Barnes' position will "make the job of the Soviet analyst or Soviet planner easier."

Barnes seemed to relish the added attention the conservative assault provided him, particularly as he enters the critical stages of his campaign for the U.S. Senate seat being vacated by retiring GOP Sen. Charles McC. Mathias Jr. He called his own press conference in the late afternoon, and then appeared with Gingrich on the MacNeil-Lehrer Newshour, calling the charges against him "nonsense" and issuing a challenge of his own -- to debate White House communications director Patrick Buchanan.

"It was Mr. Buchanan who started this thing, after all," Barnes said at his news conference.

After 10 minutes of heated exchange on the news show, Barnes said pointedly to Gingrich: "Let's stop calling people names."

In recent weeks, President Reagan has made personal appeals urging members of Congress to vote next week for the aid package, which includes $70 million in military support for the rebels, or contras, who are attempting to overthrow the ruling Sandinista government.

Gingrich said earlier yesterday that Barnes was singled out for criticism because of his comments last week comparing the tactics of the Reagan administration to those of the late Sen. Joseph R. McCarthy (R-Wis.), and because he is "one of the leading advocates of the radical liberal world view of Central America."

Gingrich said that the better that Maryland voters "get to know Mike Barnes, the fewer votes he'll get," and added that the attention won't help the Montgomery County Democrat.

"I think that if to become known in the state of Maryland as the man who did more than anybody else to stop freedom in Central America is useful [to Barnes], then Maryland is different than I think it is," he said. "I think that this whole debate shows Barnes to be an ideological radical in his world view."

Barnes is also under attack by the National Conservative Political Action Committee (NCPAC) and the Maryland Young Republicans because of his opposition to contra aid.

One congressional source said that conservative Republicans have become increasingly upset that Barnes' record on Central America has not been an issue in the Senate race. "They just want his record out. They want to make sure he does not get a free ride," he said.

A Gingrich aide said that the Georgia congressman was in his home district Monday when he decided that conservatives needed to strike back at Barnes.

Gingrich talked with Walker by telephone and then met with Hunter on Tuesday in Hunter's congressional office. Together they drafted a letter, and then Gingrich made a short speech on the House floor criticizing radical and liberal Democrats without attacking Barnes by name, which would violate House rules.

Some Republicans and Democratic supporters of other Maryland Senate candidates, however, cautioned yesterday that the attacks might help Barnes, not hurt him. One supporter of Rep. Barbara A. Mikulski, a Baltimore Democrat who is the current favorite in the race, said recently that the conservative reaction could make Barnes, who is struggling for visibility, better known among voters.

Another Democrat pointed out that NCPAC attacks against Sen. Paul S. Sarbanes (D-Md.) in his 1982 Senate race backfired and were thought to have contributed to his easy victory.