The first Harris Survey soundings of the 1978 political season show that Democrats have moved to a substantial 49-to-28 percent lead in the race for seats in the House of Representatives.

In off-year congressional elections, the party in power usually loses seats, and this year promises to be no exception. But these early readings on the 1978 race suggest that, at least initially in this election year, Democratic losses could well be relatively modest. In turn, this would put the burden of making gains on the Republicans.

The main reasons for this are not hard to find. Traditionally, normal party preferences take over on that line on the ballot where the candidates for the House of Representativs are listed. On this basis, the Democrats hold a 43-to-20 percent edge over the Republicans, with another 27 percent of the electorate classifying themselves as independents. Although independents favor Democrats over Republicans in their districts by a relatively narrow 38-to-26 percent, the 2-to-1 ratio that the Democrats begin with gives them a substantial advantage.

However, there is one other finding in this recent Harris Survey of 1,882 Americans that might well mean trouble for the Democrats. In 1977, when people were asked to rate the job being done by their own member of Congress, in contrast to the performance of Congress as a whole, a 40-to-22 percent plurality gave their representative positive marks.

In 1978, this appears to have changed. By 43-to-39 percent, a plurality now gives their own member of congress a negative job rating. In this year's race, the Democrats will have twice as many incumbents running for reelection to the House as the Republicans have. This means that the negative reactions to incumbents could hurt the Democrats twice as much as the Republicans.

At this time, the heart of the Democratic vote is drawn from those voting segments which have given that party its majorities in the past:

Those who view themselves as Democrats go 75-to-8 percent for their party's candidate for Congress.

Blacks register a thumping 77-to-9 percent preference for the Democrats.

Roman Catholics are running 50-to-25 percent proDemocratic.

Big-city voters express a 56-to-24 percent preference for Democratic candidates.

Those in the $5,000-to-$10,000 income brackets are 58-to-25 percent Democratic.

By the same token, the Republicans have some pockets of strength, although they are almost uniformly trailing the Democrats:

In the Midwest, the Democratic lead is only 46-to-35 percent, indicating that this section is the most promising for the GOP candidates for Congress.

In the suburbs, the Democrats hold a 46-to-30 percent edge, but in rural areas it is a relatively close 42-to-36 percent.

Among business executives, these first results show a 39-to-39 percent standoff between the two parties.

Among white Protestants, the Democrats hold a tenuous 38-to-37 percent lead.

Among the 20 percent of the electorate who view themselves as Republicans, the GOP holds a solid 79-to-8 percent edge.