CHICAGO, SEPT. 4 -- Although the Republican Party under President Reagan has made tremendous gains, the continuing invulnerability of Democratic members of the House has served as a roadblock to the kind of partisan realignment that took place in the late 1800s and the 1930s, according to a number of political scientists and pollsters gathered here.

As both major parties gear up for the 1988 presidential election, participants in the 83rd Annual Meeting of the American Political Science Association voiced ambivalent and often conflicting assessments of the prospects for the Democratic and Republican parties, although no one disputed the basic findings of Richard Wirthlin, a GOP pollster:

"There was a great shift in partisan identification" after Reagan won in 1980, but six years later, "that change has remained virtually intact," with no sustained growth or decline since 1981. Wirthlin's data, strongly supported by findings from CBS polls, showed that a pre-1981 Democratic advantage of 22 to 25 percentage points over the GOP has fallen to 5 to 7 points.

The inability of the Republican Party to convert the Reagan victories in 1980 and 1984 and the 1980 Republican takeover of the Senate into a partisan breakthrough -- a realignment in which the GOP gains majority status and full control over the national legislative agenda -- results in part from the near invincibility of incumbent House Democrats, according to David W. Brady, a Stanford University political scientist, and a number of others at the meeting here.

Brady contended that the change in voter attitudes in 1980 was roughly as strong as the shifts in 1860-64 and 1896, two previous realignment periods. However, the inability of the Republicans to gain control of the House -- or keep control of the Senate -- leaves them without the institutional power necessary to achieve a full-fledged partisan conversion of the electorate.

The GOP in 1980 picked up about 5 percentage points among voters casting ballots in House contests, about the same as in the Civil War and 1896 realignments when the GOP gained firm control of Congress. The number of competitive seats in the House, however, has dropped so sharply during the 20th century that GOP takeover currently is impossible without a voter shift of a much larger magnitude, Brady said.

He presented figures showing that the percentage of competitive House seats -- those with elections resolved by margins of 5 percentage points or less -- fell from 39.2 percent in 1860 and 34.5 percent in 1896 to 15.7 percent in 1980.

A similar, although more extreme, argument was made by Walter Dean Burnham, of the Massachusetts Institute of Technology, who contended that declining voter turnouts and the sharply increasing number of split-ticket voters suggest that politics has become increasingly irrelevant to the voters. "The partisan-organized political system . . . has ceased to exist in our own time," he said.

To substantiate what he considers to be a continuing "decay" of partisan commitment among voters, Burnham said the 1986 elections produced the lowest turnout levels outside the South since 1798, and that the 1984 election produced an unprecedented number of congressional districts supporting Democratic House candidates while casting majorities for Reagan.

While Burnham described a sharp decline in the importance of political parties to the general electorate, a number of other findings were presented here:David J. Lanoue, of Illinois State University, conducted a study of presidential popularity during periods of rising inflation and during recessions. He found that Republicans can recoup losses from recessions far better than Democrats can regain support lost as a result of inflationary policies.

"One of the strongest distinctions between the American parties is over macroeconomic policies . . . . Democratic presidents concentrate on lowering unemployment even at the expense of higher inflation rates, while Republicans do the opposite," he wrote. In the case of recessions under GOP administrations, however, losses in popularity are generally restored during recoveries, while for Democrats, popularity lost when inflation rises "is not reversed" when inflation is halted. "No relief {for Democrats} comes when the inflation rate drops." While tensions between fundamentalist Christians and party regulars appear to have been growing in the Republican Party, two Furman University political scientists, John C. Green and James L. Guth, contended that conflicts among GOP activists appear less likely to result in the kind of politically fatal, internal conflict that has periodically characterized the Democratic Party.

On the basis of extensive survey data on Republican contributors, Guth and Green found these activists to be split into "hard right," traditional conservative, moderate and libertarian factions, but among these competing groups "there is significant overlapping" in ideology and political style . . . there are firm grounds for strong and cohesive coalitions."

Lyman A. Kellstedt, of Wheaton College in Illinois, countered that Guth and Green "underplay the fight" in the GOP, citing the already bitter contests between Christians mobilized by Marion G. (Pat) Robertson and party regulars generally affiliated with Vice President Bush in such diverse states as Iowa, Michigan, South Carolina and Texas. In the key state of Texas, David B. Hill, of the Republican polling firm of Tarrance, Hill, Newport & Ryan, suggested that the strong gains of the Republican Party may be slowed by feuds between competing factions, particularly the evangelical Republicans "who regularly complain of being left out of party affairs."

He presented charts, however, that showed a strikingly strong correlation between Republican gains in the Texas legislature and the rising turnout in GOP primaries. The figures suggest that the GOP could significantly benefit if it can persuade large numbers of independents and conservative Democrats to participate in the Republican presidential nomination contest on March 8, the southern "Super Tuesday."

Although Wirthlin, whose firm has conducted 16,000 to 20,000 interviews a year since Reagan took office, found that the strength of the Republican Party has remained relatively flat during the Reagan administration, his findings show considerable change in the basic GOP coalition.

From 1981 to 1987, the GOP has shown its largest gains among young voters: 4 percentage points among those 17 to 24 and 5 points among those 25 to 34. In contrast, elderly voters have moved sharply away from the GOP: a 14-point drop among those 55 to 64 years old and 13 points among those over 65. Throughout the Reagan presidency, the Democratic Party has charged that Social Security benefits are likely to be cut by Republicans.

Among different income groups, Wirthlin found that the already sharp disparity between pro-Democratic poor voters and pro-Republican affluent voters became larger between 1981 and 1987.