The Democratic core of this southern California congressional district is crumbling beneath Rep. George E. Brown Jr. (D-Calif.), a cigar-smoking 64-year-old who won his political spurs fighting to end the Vietnam war.

The Kaiser Steel Co. plant, with 8,000 unionized workers, folded two years ago, along with a General Electric manufacturing plant employing 1,500. The airport in once-Democratic Ontario is attracting nonunion high-technology industry, replacing the railroad-based economy of neighboring San Bernardino.

New tract housing has sprouted throughout the district, producing a subculture of commuters to Los Angeles and Orange County.

"We have 270,000 voters in the district; 90,000 of them are under 40 and registered since the 1980 election," Tim Lynch, Brown's campaign manager, said. "That's the group our opponent is counting on. They have been breaking his way."

Both national political parties have battled intensely this year for new voters.

As a result, the partisan balance of power has changed in marginal congressional districts. In most districts, a combination of nonpartisan registration and poor record-keeping makes it difficult to determine which party is benefiting from new registration.

In such states as Florida, Pennsylvania, California and Ohio, a sampling of districts where data are available suggests, however, that Republicans are edging ahead.

Here, in California's 36th District, there is a slight but significant tilt toward the GOP. In plain numbers, Democrats would appear to have an edge. Since the May primary, Republicans have registered 20,251 new voters and Democrats 21,972.

But in California, where party allegiance is weak, the key figures are percentages. The GOP, the minority party, has grown from 30.3 percent of the voters in the 36th District to 32.9 percent, while Democrats have declined from 57.5 percent to 56 percent, a shift of 4 percentage points.

"Thirty-five has always been the magical number," said John Rico, political director of the California Republican Party. "Everybody looks out and says if a district goes over 35 percent, we ought to win it." By that standard, the GOP is 2.1 percentage points shy of the magic number in the 36th District.

New voters are splitting evenly between the two parties, a bad sign for Democrats because Democratic registrants have proved to be less loyal than Republicans. Also, the 36th District's new voters do not appear to be drawn to Democratic tradition.

"On economic issues, they basically have the attitude, 'I made it on my own and everyone else can, too,' " Lynch said. "The programs that got people into the middle class were the helping hand of government, but they take it for granted. They have no sense of history. On that one, we lose."

Norman Turnett, campaign manager for John Paul Stark, Brown's GOP challenger, sees the new voters from a different vantage point. "This is the new Orange County," he said, referring to the intensely conservative county to the south.

In Orange County, the GOP trend is sharper than in Brown's district. If breaking the 35 percent mark is critical, the registration trends in the one Democratic enclave of Orange County -- the 38th District -- suggest serious problems for another Democratic incumbent, Rep. Jerry M. Patterson.

In the Orange County section of the 38th District, which produces 80 percent of the vote, the GOP share rose from 35.4 percent to 39 percent while Democratic registration dropped from 53.1 to 50.1 percent -- a shift of 6.6 percentage points in six months.

Patterson has been challenged by former representative Robert Dornan, a Republican who only recently moved into the district, first giving as his residence his campaign headquarters and then a Holiday Inn. Despite Dornan's possible vulnerability as an outsider, he generally is given an even chance of winning on Nov. 6.

"They Republicans had paid registrars in shopping centers, everywhere," Patterson said.

"From a strictly Democratic point of view, I think you could make a case that it Democratic strength is eroding," Patterson aide Kurt Haunfelner said. "Quite frankly, our registration effort was too little too late."

The registration drives in the 36th and 38th districts suggest that key factors were not so much a partisan shift of the electorate, but the time, money and effort each party spent.

In California, the state GOP, backed by the Republican National Committee, put together a $1 million voter registration program. The party paid a firm to place registrars in shopping centers throughout the state and gave GOP organizations a $1 bounty for every Republican voter they signed.

Before the registration drive, two of the state's 58 counties had Republican pluralities. Afterward, seven counties had GOP pluralities, including San Diego County, which went from a Democratic advantage of 10,562 last May to a Republican advantage of 25,333 by Oct. 6.

This sort of GOP shift was even sharper in sections of Florida. Democratic registration in Miami's Dade County decreased between 1982 and 1984, from 487,448 to 448,978, while GOP registration grew from 162,613 to 211,413.

In Palm Beach, the minority Republican Party grew by 22,000, compared with an increase of 20,000 among Democrats. But, more importantly, the GOP made a net 3-percentage-point gain.

These shifts could prove important in the Palm Beach race between Rep. Daniel A. Mica (D-Fla.) and his GOP challenger, Don Ross, and in the southeast Dade County contest between Rep. Lawrence J. Smith (D-Fla.) and Tom Bush, although both incumbents are expected to win.

In Pennsylvania, a check of three competitive House contests shows a near tie between GOP and Democratic registration gains in two of the districts and a modest Democratic gain the other.

In the 7th District of Pennsylvania, where Curt Weldon is challenging Rep. Bob Edgar (D-Pa.), and the 8th District, where David A. Christian is challenging Rep. Peter H. Kostmayer (D-Pa.), registration has grown significantly. But the ratio of Republicans to Democrats remains almost what it was in 1982.

A potentially significant Democratic gain came in the 21th District, the Erie area, where the Rep. Thomas J. Ridge (R) won the seat by 729 votes in 1982.

Since then, Democratic registration in the 21st District has grown by 11,360 while the GOP has gained 4,284 voters, and the Democratic margin has grown by 2 percentage points. If the race between Ridge and Democrat James A. Young proves tight, the registration shift could become critical.

One endangered Democrat, Rep. Joseph G. Minish (D-N.J.), may benefit from Democratic efforts. In Morris County, which dominates his district, Democratic registration has grown by 2,801 while Republican registration has declined by 1,273. In a political wild card, however, the largest growth, 26,786, has been among independents.