The 1990 midterm campaign, marked by seething voter discontent with government, seems likely to produce statistically small but politically significant shifts in the two-party balance of power, a roundup of races in all 50 states indicates.

With the Senate headed for a status quo year and Democratic House gains unlikely to top a dozen, the main losses for President Bush and the Republicans figure to come in key state gubernatorial battles. Democrats are poised to capture Florida and are serious threats in Illinois and Texas, while Ohio and possibly Massachusetts offer offsetting opportunities to the GOP. The California governorship, the top political prize in the nation this year, seems likely to remain in Republican hands.

Overall, Democrats seem ready to add a handful of governorships to their current 29 -- an important factor heading into the 1991 legislative and congressional redistricting wars.

Reports from party leaders, political observers, pollsters and campaign consultants suggest that this year's voting results will not fully reflect the loud dissatisfaction voters have been expressing about higher tax burdens and prolonged policy disputes in Washington and many state capitals.

If the sour mood and Bush's declining popularity depress the turnout of younger and more casual voters, as many GOP strategists fear, then an electorate dominated by older, New Deal-era Democrats could cost the Republicans a net loss of two Senate seats, as many as five governorships and 16 House seats.

Insiders in both parties cautioned that the evident anger and frustration among voters may well produce more than the usual number of surprises Tuesday. Particularly vulnerable are well-entrenched incumbents who are unaccustomed to competitive races and did not bestir themselves unduly this year.

The warning signals of a "throw-the-bums-out" psychology erupted early enough for people like Sens. Mark O. Hatfield (R-Ore.), John F. Kerry (D-Mass.) and probably Rudy Boschwitz (R-Minn.) to gear up strong responses. A late surge against Sen. Mitch McConnell (R-Ky.) sent him scrambling. But in House races, where polling is often more episodic, a few incumbents likely will be caught napping.

Nonetheless, the "free fall" some Republican strategists talked about in early October, when Bush's daily reversals on budget policy scuttled underlying support, seems to have been arrested in the final 10 days of the race. As Bush focused more rhetoric on his role as commander-in-chief of U.S. military forces confronting Saddam Hussein, his personal support -- and backing for GOP candidates -- seemed to steady.

Still, the midterm election is taking place in a time of pessimism about economic prospects and as more and more voters say that government's inability to function is the main reason they think the country is headed in the wrong direction.

In that environment, while all incumbents are in some jeopardy, it is those of the president's party who are likely to take the worst hit.

But a swing against incumbents is likely to be kept smaller by the huge advantage that House and Senate members enjoy in their fund-raising.

In the Senate races in particular, many of the well-known incumbents who appeared to be in jeopardy at one time or another now seem headed for reelection victories. The "October surprises" that threatened Hatfield, Boschwitz and Kerry have faded if not disappeared. The neophyte challengers in these races have faltered in the final days as the incumbents brought the heavy artillery of their experience to bear.

Earlier, Republicans had hopes against such incumbents as Paul Simon of Illinois and Carl M. Levin of Michigan and Democrats talked up chances of beating McConnell and Larry Pressler of South Dakota. But none of those threats looks highly plausible today, and some have clearly vanished.

On this final weekend, the closest Senate battles are the North Carolina race between Sen. Jesse Helms (R) and former Charlotte mayor Harvey Gantt (D), who would be the first black senator from the South since Reconstruction, and the Hawaii contest between appointed Sen. Daniel K. Akaka (D) and Rep. Patricia F. Saiki (R).

Should those races split and the others go as expected, Democrats would retain their present 55-to-45 majority but remain vulnerable for 1992, when they have six more seats to defend than the GOP.

Because 14 of the 36 governors up this year chose or were forced by state law to retire, and because those governorships are vital to the redistricting battles of next year, the state races became a major target for both parties.

Republicans have the inside track in the nation's No. 1 contest, with Sen. Pete Wilson (R) leading former San Francisco mayor Dianne Feinstein (D) for the California governorship. If Wilson succeeds retiring Gov. George Deukmejian (R), it will give the GOP leverage it did not have 10 years ago when a Democratic gerrymander cost Republicans several seats in the largest House delegation. That delegation will grow again in 1992 by a projected seven seats.

The rest of the gubernatorial news is not nearly as bright for the GOP. Despite a clear, underlying Republican trend in Florida, Gov. Bob Martinez (R), weakened by his battle for restrictive abortion laws, is losing his reelection race to former senator Lawton Chiles (D).

Even more disquieting to the Republicans, their colorful Texas gubernatorial nominee, rancher-businessman Clayton Williams, has created so much consternation with his boot-in-the-mouth comments that he could end up losing to state Treasurer Ann Richards (D).

Democrats are also a threat in Illinois, where retiring Gov. James R. Thompson (R) has kept them at bay for 14 years. State Attorney General Neil F. Hartigan (D), taking a leaf from Bush's 1988 playbook, has vowed to oppose continuation of a temporary 20 percent surtax that his opponent, Secretary of State Jim Edgar (R), supports. Hartigan's anti-tax stance may cut deeper into the Republican suburbs than Edgar can infiltrate into the Democratic base vote in Chicago. The polls are very close.

The best chance for a big-state GOP pickup is in Ohio, where former Cleveland mayor George Voinovich (R) is apparently holding on to a long-sustained lead over state Attorney General Anthony Celebrezze (D). Weariness with the Democrats after eight years under retiring Gov. Richard F. Celeste (D) seems to be offsetting discomfort with Voinovich's antiabortion stand -- which Celebrezze has tried to turn into a major issue.

Somewhat to their surprise, Republicans are also in the picture in Massachusetts, where former U.S. attorney William F. Weld (R) is closing strongly against Boston University president (on leave) John R. Silber (D). Until the last few days, Silber seemed nearly certain to take over from battered and retiring Gov. Michael S. Dukakis (D). But private polls show liberal Democrats -- particularly women -- are choosing Weld in sufficient numbers to make the race close.

An oddity of the gubernatorial picture is that in two states where the Democratic incumbents are retiring, longtime Republicans running as independents may deny the official GOP nominees victory. That is the case in Alaska, where former governor and interior secretary Walter Hickel is leading the three-way race, and in Connecticut, where former senator Lowell P. Weicker went into the weekend ahead of his major-party rivals.

In House contests, Democrats again have recruited the stronger candidates in most of the open-seat races, and look to profit in places like Arkansas, Connecticut, Hawaii, Idaho, Iowa and Rhode Island, where GOP House incumbents decided to seek statewide office.

The other likeliest turnover situations involve members who have run into criticism of their personal ethics or ties to savings and loans and other contributors. More of that damage, too, is falling on the GOP side of the aisle.

But the net loss to the Republicans is limited by the fact that Bush provided coattails for very few House Republicans in 1988 -- an underlying factor in the GOP rebellion against his budget compromise, which sent Republican prospects into decline a month ago.

One man who stuck with Bush in that ordeal but later declared his independence -- freshman Rep. Peter Smith (R) of Vermont -- is losing his race to independent Bernie Sanders, the former socialist mayor of Burlington, who says he would caucus as a Democrat.

Staff writer Maralee Schwartz and political researcher Bruce Brown contributed to this report.