Washington — The United States House of Representatives, which has been steadfastly Democratic for nearly all of the last half century, shows few signs of dramatic shifts in next fall's elections.
''There are truthfully fewer competitive races this year than two years ago, '' concludes Neil Newhouse, political director for the US Chamber of Commerce. He says the new crop of challengers has been slower off the mark and less successful at raising money than in 1982, when redistricting and retirements left 58 seats open.
So far only 21 of the 435 House seats are open in the 1984 elections, because their occupants are leaving government or running for higher office.
Rep. Guy Vander Jagt of Michigan, chairman of the national Republican Congressional Committee, who has been given to making extravagantly optimistic predictions in past years, is offering no numbers this time. ''I'd be dumbfounded if we didn't gain'' is as far as he will go.
The chairman of the Democratic Congressional Campaign Committee, Rep. Tony Coelho of California, says: ''I don't see the House changing much. We'll gain four or five seats.''
Because the Democrats now hold a 100-seat advantage, it would take a 30-to-40 seat switch to alter the strong Democratic hold on the House. But despite the mathematics, both parties are preparing their rhetoric for what could yet be a heated national House campaign. And both sides are focusing on Ronald Reagan.
The Democratic Congressional Campaign Committee fired its first shot by producing a 114-page book listing the 1980 promises of candidate Reagan - most of them marked ''broken.''
''We're trying to unmask Ronald Reagan,'' Mr. Coelho said in an interview. ''We're humanizing him.'' He added that he has amassed a videotape collection of Mr. Reagan and plans to make it a major part of the national effort for Democratic House members.
Democrats and Coelho hold that polarization over the President will bring a record voter turnout, and since the majority of Americans are Democrats, that will help Democratic House candidates.
Far from avoiding the Reagan issue, Republican Vander Jagt is charging into the fray. His committee will sound a rallying call to Reagan supporters, he says , asking voters: ''Do you want to continue the new direction launched by Ronald Reagan in '80, or go back to the Democratic policies of the past with Jimmy Carter and Walter Mondale? If you want to go in the new direction, elect Republicans.''
''When the dust has settled, and we total up the votes,'' says Vander Jagt, the results of the 1984 House elections ''will send a message to the House as to which direction the people want to take America.''
Republicans reversed taxing, spending, and military decline by taking the White House and Senate as well as making big gains in the House in 1980, the Michigan Republican notes. ''We'll decide whether the '80 changes were an aberration.''
Although House races are the last to capture the attention of voters before election day, some trends are clear. Both parties are concentrating on races for open seats, such as the three in Texas. And freshmen will be the prime targets, since they are considered the most vulnerable to national moods.
In 1982 Democratic challengers tossed out 13 of the GOP's freshman class of 1980, largely because of discontent over the recession. This year the Democrats have 57 first-termers to protect, several in ''swing'' districts that could tilt in either direction. The Republicans have only 24 freshmen, and most are in safe districts.
Thus the battle for House seats will be especially heated in North Carolina, Virginia, Pennsylvania, and Illinois, all states where Democrats made big gains.
Coelho maintains that the new Democrats will be less vulnerable than Republicans were two years ago, however, because they have stayed independent of the House Democratic leadership and Speaker Thomas P. O'Neill Jr. of Massachusetts. The freshmen have been known to turn floor votes against spending bills into symbolic protests of high deficits. ''The public likes you to establish your independence,'' Coelho says. ''Tip gets after me,'' he quips. ''He says, 'Your class is giving us all sorts of trouble.' I say, 'Yeah, but they'll get reelected.' ''
Among the close races for House seats:
* In North Carolina, Democrats soundly defeated Republicans there and now must defend their gains. The tightest race appears to be Democratic freshman James McClure Clarke's rematch with Bill Hendon, the Republican he unseated by a razor-thin margin.
* A Republican effort to knock off a Democratic leader has produced a serious challenge to House Budget Committee chairman James R. Jones of Oklahoma. Two Republicans are vying for the nomination, and they will have plenty of funding from the GOP, which has targeted Representative Jones.
Democrats would like to counter with a challenge for the House Republican whip, Trent Lott of Mississippi. They are trying to coax former Lt. Gov. Evelyn Gandy, who ran for governor, into the race.
* Freshman Democrat Bruce A. Morrison of Connecticut will face the former occupant of his seat, Rep. Lawrence DeNardis, in a campaign that has never really ended since the Republican lost his seat in 1982.
* Rep. Thomas R. Carper (D) of Delaware won his seat in 1982 largely on the strength of a scandal involving the Republican incumbent. Now he faces a GOP candidate with guaranteed name recognition, Elise du Pont, wife of Gov. Pierre S. du Pont IV.
* Veteran Democrat Les Aucoin of Oregon faces a rematch with Republican Bill Moshofsky, a former lumber company lobbyist, who nearly won in the last contest. Mr. Moshofsky will be guaranteed generous financial backing from Republicans who have targeted the Oregon seat.
* Freshman Republican Webb Franklin will probably have a difficult battle to retain his Mississippi Delta seat. A court order has redrawn the district lines to include 5 percent more black residents, or 53 percent of the voting-age population. Three black Democrats are competing for the nomination, and the best known is State Rep. Robert G. Clark, who ran in 1982.
* In races involving members tainted by scandal, three members face a clutch of challengers. Rep. Daniel B. Crane (R) of Illinois already had a loose grip on his Bible-belt district, but his reelection is in doubt since he was censured by the House for sexual misconduct with a teen-age congressional page.
Rep. Gerry E. Studds (D) of Massachusetts, censured for his sexual involvement with a young male page, also has primary opponents and a list of GOP challengers.
Idaho Rep. George Hansen (R) has a difficult reelection battle due to his indictment by a federal grand jury on charges of falsifying financial disclosure forms.
STATES WITH FRESHMEN UP FOR REELECTION Dem. Rep. Alabama 1 Arizona 1 1 California 8 3 Colorado 1 Connecticut 1 1 Delaware 1 Florida 2 3 Georgia 4 Illinois 4 Indiana 2 1 Kansas 1 Maine 1 Michigan 1 Minnesota 2 Mississippi 1 Missouri 1 Nevada 1 1 New Jersey 1 New Mexico 1 New York 4 1 North Carolina 3 Ohio 2 2 Oregon 1 Pennsylvania 3 2 South Carolina 2 Tennessee 1 1 Texas 5 1 Utah 1 Virginia 3 1 Washington 1 West Virginia 3 Wisconsin 1