The Republican Party and friendly interest groups plan to pour $500,000 into campaigns against each of 40 incumbent Democratic congressmen this year, but the GOP has all but given up hope of taking over the House, top White House aides said today.

Claiming he hated to be a "party pooper," White House chief of staff James A. Baker III said that Republicans have lost an average of 38 seats in each off-year election, and it "would be a little short of a miracle for the party to gain control of the House.

"I'm not saying 1982 elections are a lost cause. I'm saying we have to be realistic," he said tonight. "It's going to be an uphill battle; our goal must be to solidify control of the Senate."

White House political director Ed Rollins said the administration plans to "throw all our resources" into campaigns against 40 Democratic House members, whom he did not identify.

To gain control of the House, the Republicans would need to pick up 26 seats.

"We feel we can knock off 10 to 15 of these members. The key to our success is how many of our own can we keep," he said.

Democrats now outnumber Republicans, 243 to 192, in the House but the Republican Party has assembled an unprecedented war chest and a sophisticated campaign apparatus to reduce that margin.

Under election law, the Republican National Committee and state party organizations each can contribute $5,000 to a House candidate in a primary election, $5,000 in a general election and an additional $18,210 in "coordinating expenses."

The Republican Congressional Campaign Committee is allowed to spend an additional $5,000 in the primary and $5,000 in the general election for each candidate.

Rollins said he expects friendly independent political action committees will donate an additional $500,000 in each of the 40 targeted races.

Rollins said he expects Democrats to match Republicans dollar for dollar in many races. There is no limit to how much money a congressional candidate can receive.

Rollins, who succeeded Lyn Nofziger as political director, made his comments at a southern Republican leadership conference here in which an undercurrent of fears about the nation's economy dampened otherwise optimistic election-year hopes.

The Reagan administration sent three Cabinet members, Interior Secretary James G. Watt, Labor Secretary Raymond J. Donovan and Housing and Urban Development Secretary Samuel R. Pierce Jr., Baker, and national party chairman Richard Richards here to give pep talks to the party loyalists.

But persistent questions about unemployment, high interest rates and Reagan's economic program kept cropping up.

Some of the most pointed remarks came from Rep. Jack Kemp (R-N.Y.), coauthor of the Kemp-Roth tax cut bill and a longtime Reagan supporter.

In private meetings with state party leaders, Kemp directed harsh criticism at budget director David A. Stockman and his budget projections.

In public, he was somewhat muted. But in a speech this morning, Kemp called for resignation of Federal Reserve Chairman Paul A. Volcker and criticized those in the administration who have been lobbying for tax increases.

"I can't imagine raising taxes in 1982 and going before the American public in elections next fall," he said.

Other ripples of discontent surfaced during the two days of meetings attended by 1,000 delegates from 13 southern states.

James Bacon, an unsuccessful 1980 congressional candidate from Alabama, complained at one session today that unemployment in his state now stands at 14 percent.

"In my opinion it will be pretty disastrous for Republican candidates if the situation doesn't get cleaned up by election day," he said.

Thomas Green, a Gainesville, Fla., small businessman, complained "these high interest rates are just killing us."

"I still believe in Ronald Reagan," he added. "But how am I going to have money and time to support Republican candidates when my business is floundering."

Administration spokesmen tried to divert the criticism by blaming the problems on past actions of liberal Democrats.

This worked for the most part.

"It took us 40 years to get into this fix and you don't get out in 14 months," said Henry B. Sayler, Florida GOP chairman.

But Sayler admitted some uneasiness. "We're all hoping that the economic trends are moving in the right direction by fall. If they are, we're going to win big. If they aren't, we're in trouble."

Republicans at the conference were briefed on the most elaborate Republican election plans in history.

They include spending about $18 million in direct aid to Republican House and Senate candidates this year, and tens of millions dollars more in indirect aid, such as polling and research.

Tennessee Gov. Lamar Alexander had some sober words for those interested in state and local races, however. He said traditional Republican national issues such as a strong national defense and reduced government spending do not much help candidates in these races.

"Jumping up and down about federal spending and the gold standard isn't going to help a Republican candidate for governor in a southern state," Alexander said.

Voters in these races are interested in improving education, roads, sewers, the environment and preserving the southern way of life, he said.

"You might be saying all of this sounds more like a Democratic platform than it does a Republican platform. Well, if it does, I would submit that is a problem for Republicans, not for Democrats."