The 1990 midterm election today will bring to an end a volatile campaign in which Democrats grabbed control of the issue agenda but seem likely to reap only modest gains in the voting booth.

"The Democratic Party has gone a long way this year toward reestablishing itself as the party of the working class and tying the 'party of the rich' label back around Republicans' necks," said Republican pollster Richard Wirthlin.

"The Republicans have been in retreat and disarray," agreed Norman Ornstein, a political scientist at the American Enterprise Institute. "It's not merely the political ineptitude of President Bush on the tax issue these past few weeks. It's larger than that.

"They're victims of their own success," Ornstein said. "The demise of communism has hurt them. So has the fact that judges they appointed have changed the dynamics of the abortion debate. So has the fact that, after you've been in power so long, it's hard to keep running against Washington."

Since World War II, the average gain for the party out of presidential power in midterm elections has been 28 House seats and four Senate seats. This election Democratic operatives say they will be happy to win 10 House seats and one Senate seat -- in part because they already have record high congressional representation for an out-party in a first presidential term; in part because, even in a year of voter disenchantment with politicians, incumbents enjoy such lopsided financial advantages that they are difficult to dislodge.

Democrats now control the Senate by 55 to 45. Thirty-four states will elect senators today; 16 of those seats are held by Democrats; 18 by Republicans. In the House, where Democrats enjoy a 258 to 175 advantage (with two vacancies), all 435 seats are up for grabs, including 29 (18 Republican-held; 11 Democratic-held) in which the incumbent is not seeking reelection.

Of the 36 states that will elect governors today, 20 have Democratic chief executives and 16 have Republicans.

Late tracking polls showed that two of the most endangered Senate GOP incumbents, Oregon's Mark O. Hatfield and Minnesota's Rudy Boschwitz, opened up late leads, apparently on the strength of heavy eleventh-hour negative advertising attacks against their opponents. And in North Carolina, the most expensive Senate race in the nation, Sen. Jesse Helms (R) closed his campaign with an ad accusing Harvey Gantt (D), the former Charlotte mayor who is seeking to become the first black senator from the South since Reconstruction, of supporting racial hiring quotas.

In the governors' races, late polling data brought good news for Republican nominees in Massachsuetts, where William F. Weld was closing in on Democrat John R. Silber; and in California and Ohio, where Sen. Pete Wilson (R) and former Cleveland mayor George Voinovich (R) were maintaining their small leads. Republicans are hoping to hang onto the governorship in Texas, where GOP nominee Clayton Williams closed his campaign with an ad in which he obliquely apologizes for the many gaffes he committed during a bitter campaign against state Treasurer Ann Richards (D).

The best Democratic hopes for big-state gains now appear to be in Florida, where polls show former senator Lawton Chiles (D) with a small lead over Gov. Bob Martinez (R), and in Illinois, where Democratic Attorney General Neil F. Hartigan has pulled even with Republican Secretary of State Jim Edgar.

Whatever the numerical shifts, Democrats hope that the 1990 election will serve as a thematic and political bookend to the 1978 midterm. That year Republicans were the out-of-power party, and they put forward a new economic platform built around cutting taxes and downsizing the role of government. While they won some races in 1978, their agenda jelled in a much bigger way in 1980 -- when Ronald Reagan was elected president and the GOP picked up 34 House seats and 12 Senate seats, including those held by a handful of the Senate's most entrenched liberals.

Democrats hope that, in much the same way, their economic theme of 1990 -- tax-the-rich populism -- will gather force between now and 1992. But there are plenty of skeptics.

"They'll find themselves mouse-trapped if they think that taxing the rich is going to get them the White House," said William Schneider, a visiting professor of Amercian politics at Boston College. "That's an oppositon slogan, but it isn't a governing program. Democrats who think it's a governing program ought to look very hard at the experience of governor Jim Florio {D}," who has seen his approval ratings tumble in New Jersey this year after he tried to shift the tax burden from the poor to the upper-middle class and the wealthy.

Beyond economics, 1990 has seen has sharp change in the social-issue terrain. Bush's successful 1988 campaign included a skillful manipulation of the "flag" issue against Democratic presidential nominee Michael S. Dukakis. In 1989 and again in 1990, the president pushed a constitutional amendment to ban flag desecration -- only to be beaten back by a Democratic Congress. Many Democrats feared their votes would be used against them this fall.

"That was going to be the Republican hot-button issue six months ago, but it was nowhere to be found on the campaign trail," noted Geoffrey Garin, a Democratic pollster. "With a recession looming, the pocketbook issues have trumped the social issues."

A similar dynamic has overtaken the politics of abortion. A year ago, following the Supreme Court's Webster decision opening the door to more state restrictions on abortions, many predicted the issue would dominate the 1990 campaigns. But a soft economy and the deployment of U.S. troops in the Persian Gulf have diminished its impact.

Still, abortion is another issue in which the terrain has shifted in ways favorable to Democrats, whose candidates generally are more supportive of abortion rights. In the 1970s and 1980s, antiabortion candidates were most eager to raise the issue in campaigns. In the post-Webster era, with the nation's abortion-rights majority suddenly threatened by courts and state legislatures, abortion-rights candidates have been the most vocal on the stump -- and their ads have been the most graphic.

One campaign ad, made of behalf of Democrat James P. Moran Jr., who is challenging Rep. Stan Parris (R-Va.) in the Northern Virginia's 8th District, depicted women being placed behind bars for seeking an abortion. Another, made on behalf of Democrat Ron Twilegar, a Senate candidate in Idado, used the imagery of coat hangers to evoke the specter of illegal, and sometimes lethal, abortions.

Another pillar of Republican success in political campaigns over the last two decades -- anti-communism -- also has ebbed this year. There has been relatively little disscussion of defense issues in campaigns, and not much partisan division over Bush's Persian Gulf policy.

Some of the high-stakes choices voters will make have nothing to do with candidates. Nearly 70 citizen-initiated referenda, the most in seven decades, will be on state ballots around the country. Three of them -- two in California and one in Colorado -- would limit the terms of legislators. About a dozen would impose tax caps of one kind or another.