Parties quietly shuffle the voters.
The faces tell a story. The Republican convention here this week has fewer blacks, fewer Hispanics, fewer Asian-Americans, fewer American Indians, far fewer minorities overall, than last month's Democratic convention. Most folks here are white and middle-class.
Says Reagan campaign manager Ed Rollins: ''This is the most homogeneous group of delegates brought to a Republican convention in a long, long time.''
Is that bad? As long ago as 1970, the New York Times expressed concern that Republicans and Democrats might be polarizing into two ''ideologically pure conservative and liberal parties.''
More recent analysts point to figures that show Democrats drawing a greater portion of their support from black, women, and the poor, while the Republicans become the political home to those voters who are white, male, and better off.
The numbers back up some of these observations.
For instance, in 1956, when Republican Dwight D. Eisenhower won reelection, nearly 40 percent of all blacks gave him their votes. By 1980, only 10 percent went for Reagan.
This shift, however, is seen by a number of political scientists as only a natural evolution - a healthy trend in the body politic. They don't worry greatly about it.
David E. Price of Duke University, author of the new book ''Bringing Back the Parties,'' says that some of the changes taking place in the South, for example, could have been expected. Blacks have lined up almost solidly Democratic - thus forsaking their old, emotional ties to the party of Abraham Lincoln. At the same time, more and more white Southerners slipped into GOP ranks, the party that sent all those Union troops into the Confederacy during the Civil War.
Says Professor Price: ''What we are seeing is ... just a kind of working out of the last stages of the New Deal alignment. Conservatives and liberals are going where they are supposed to go.''
Reports from the Gallup Poll show the Southern black-white shift isn't the only big change. The Roman Catholic vote was once considered a Democratic stronghold. Catholics even favored Adlai Stevenson over the popular Eisenhower. Yet in 1980, for the first time, Reagan carried the Catholic vote for the Republican Party.
Another change: Labor voters have been heavily Democratic in almost every election. Yet the GOP now has loosened that Democratic grip so much that Reagan got 43 percent of the vote from union households in 1980 - and is running even stronger than that this year.
Then there are the Hispanics. Traditionally Democratic, Hispanics are a group that looks ready for courtship by the GOP. Issues like abortion, a strong military defense, prayer, and federal aid to Roman Catholic schools have all been used by Reagan to woo Hispanics. Polls indicate that the President is running about 10 points stronger with Hispanics this month than in November 1980 .
Rather than a polarization, political scientists say that the parties are juggling their support. In some cases, the effect actually is to depolarize the nation's politics.
Republicans, once mostly Protestant, today have more Catholic and Jewish support than in recent memory.
Democrats, once able to rely on a solid South, now have to fight for every Southern state - and they usually lose in presidential races. At the same time, over three decades, Democrats have won over ''silk stocking'' Protestants, many of them former Republicans.
Where does all of this leave us?
Political scientist Tom Cronin of Colorado College suggests: ''Republicans (in the South) have won majorities of whites because of patriotism and a strong defense policy, and some traditional values. They are clever in doing it. It's rational. ... We're talking 20 years of Southern strategy (by the GOP), and it has worked.''
Dr. Cronin adds, however: ''I must say I don't like the idea that we have all the blacks in one party.''
Then he continues: ''I think that will change, too. I think that as blacks ... break through economically, slow as that may be, that 30 years from now a lot of blacks ... will be in the Republican Party because of economic concerns.''
Already, suggests Cronin, Republicans are making big progress with Hispanics, in part because many Hispanics are moving toward middle income.
As for the smaller number of black, Hispanic, and other minority faces at this convention, Republican US Senate candidate Mary Mochary of New Jersey has a ready answer. Since about 90 percent of the nation's blacks consider themselves Democrats, she observes, their numbers at the GOP convention will naturally be smaller. It could hardly be any other way.
The most troublesome part of all this, says Professor Price of Duke, is neither a lack of minorities among Republicans nor a shift of whites away from the Democratic Party.
Price's prime concern is the growing number of self-styled independents. Nonaligned, independent voters are usually easier prey for single-issue pressure groups. That, in turn, undercuts America's party structure. The result is that political leaders, without a broad party base, have a more difficult time finding a popular consensus, and special interests are strengthened.