With two Democratic congressmen already in the fold, White House political strategists and Republican House leaders say that Democratic Reps. Larry P. McDonald of Georgia, Richard C. Shelby of Alabama and Ronald M. Mottl of Ohio are the most likely to switch parties next.

These GOP leaders also are maintaining close contact with about 20 other conservative Democrats in hopes that two or three more will switch parties before the 1982 election. Republican leaders further contend that as many as a dozen more will join them if the GOP picks up 12 to 15 seats next year and are within striking distance of forming a "conservative coalition" to take over the House.

Shelby went "up to the brink" of switching parties last month along with Rep. Bob Stump (D-Ariz.) when Stump announced he would run as a Republican in 1982, a House GOP leadership source said yesterday. Republican sources say Mottl is considering a party switch because his district probably will be combined with that of Mary Rose Oakar, another suburban Cleveland Democrat, through redistricting and he'd prefer to run against her as a Republican.

Both congressmen said in interviews that they have no plans to change parties, although Shelby expressed a deep bitterness over threats by Democratic leaders to discipline wayward party members.

"That threat smacks of the old post-Confederate period," said Shelby, who added he hoped to work within the party to "bring it back from the left wing to the middle of the road.

"If the Democratic Party continues to go to the left, it will destroy its southern base," he added.

"I'd rather fight than switch," said Mottl.

McDonald, an ultra-conservative, fourth-term Democrat, however, hinted broadly that he is considering changing parties. "Obviously, you can always change partners in a dance," he said. "It's up to each person to decide when the music stops."

The comments came as Rep. Eugene Atkinson, a second-term congressman from western Pennsylvania, left the Democratic Party to become a Republican because "the modern Democratic Party bears no resemblence" to what it was when he joined it 20 years ago.

At an unusual news conference in the White House Rose Garden, President Reagan welcomed Atkinson's switch, saying the congressman had shown "outstanding political courage that symbolizes a new era of politics in America."

"Gene and I both came to the realization that the party we belonged to had grown away from the concerns of the common man," the president, a former Democrat, added.

House Republicans greeted Atkinson, who supported Sen. Edward M. Kennedy in last year's Democratic presidential race, with open arms. "It's one step closer to a majority," said Minority Whip Trent Lott (R-Miss.). "We've got some other Democrats we're talking to."

Democrats took extraordinary pains to downplay the move. "Gene who?" said House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.). "You can see he's made quite a mark around here."

Atkinson, he said, had "bartered his principles for the opportunity to win in a district he cannot win" as a Democrat.

Rep. Tony Coelho, chairman of the Democratic Congressional Campaign Committee, charged Atkinson had been "politically bought out" by the Republican Party. The new Republican "has a history of changing his position and political support to further his own career," he added.

Atkinson, 54, supported Jimmy Carter in 1976, then switched to Kennedy in 1980 after Carter had spoken at a fund-raiser for him. Later, he flirted briefly with supporting independent presidential candidate John B. Anderson. Three weeks ago he told a reporter: "I'm a lifelong Democrat. I have no intention of changing."