In midsummer of a midterm election year, the Republican president sends troops to the Middle East to shore up a friendly nation against a suddenly menacing Iraq.

He delivers a nationwide television address, calling the situation "grave" and describing the military deployment as "essential to the welfare of the United States."

His approval ratings shoot up.

By November, however, voters are worried about something closer to home: a recession. The president's party suffers one of its worst defeats ever in a congressional election -- dropping 14 seats in the Senate and 48 in the House.

No one is predicting that the rout Republican candidates suffered in 1958 -- just a few months after President Dwight D. Eisenhower dispatched troops to Lebanon in response to a coup in Iraq -- will repeat itself this fall.

But 1958 offers a pair of timely reminders as the 1990 midterm campaign moves past the traditional Labor Day starting post. One is that voters have a short attention span and a shorter memory. The other is that, absent a shooting war, they vote their pocketbooks.

That is especially true when the economy goes soft. And this summer, the percentage of Americans who thought the nation is heading into a recession within a year doubled to 57 percent, according to an NBC/Wall Street Journal poll taken in mid-August as gasoline prices began to rise.

These economic jitters, combined with raw anger at the savings and loan debacle, make the 1990 campaign one of the most "potentially explosive" in recent memory, said Democratic pollster Geoffrey Garin.

But it is also one of the least predictable. Will domestic frustrations trigger a throw-the-rascals-out electoral uprising? Or will the Middle East crisis freeze incumbents in place?

"This is the first Labor Day in a long time when the course of the election and the outcome of the campaign are uncertain because of events that have not occurred yet," said Garin. "One, obviously, is Iraq and the other is the domestic budget summit."

If the Middle East continues to dominate the news throughout the fall, experts in both parties say it will hurt challengers of both parties. "In times of crisis, people stay with the status quo," noted GOP consultant Don Sipple. Not only will challengers and their agenda get crowded off the news screen, but incumbents -- especially senators in tight races who are on key foreign policy committees such as Claiborne Pell (D-R.I.), Paul Simon (D-Ill.) and Carl M. Levin (D-Mich.) -- will tend to get more positive exposure.

But if there is a tax increase as part of a budget deficit-reduction deal, it is likely to hurt incumbents, again, on a bipartisan basis. "There will be a lot of congratulations here {Washington} and an uproar everywhere else," said Garin.

One of the reasons that the politics of 1990 is so unformed is that while there's evidence of an anti-government, anti-incumbent mood -- congressional term limitation measures will be on several state ballots this fall -- the normal outlets for expressing that mood seem less accessible than usual.

Throughout American history, voters have used midterm elections to throw the "in" party out. But an "in" party cannot lose what it never won. In 1988, while George Bush was being elected president by a handsome 7-million-vote margin, Republicans lost seats in the House and Senate. Bush assumed office last year with the smallest partisan base of congressional support of any newly elected president in history -- a deficit of 10 seats in the Senate and 85 in the House.

As a result, election experts predict only small changes in the partisan makeup of both chambers. If there is an anti-incumbent wave, they say, it is more likely to strike incumbents of both parties, especially those tainted by scandal or the S&L fiasco.

But here again, there is a structural impediment to wholesale change: Incumbent advantages are more lopsided than ever before. In the House, the 98 percent reelection rate of incumbents in 1986 and 1988 made it difficult to recruit quality challengers this year. In the Senate, half of the 32 incumbents up for reelection have no effective opposition and, overall, incumbents have raised more funds than challengers by a 5-to-1 ratio.

The 36 gubernatorial races are expected to be more competitive because the issues governors deal with -- education, roads, taxes, abortion -- have a more immediate impact on daily life. Democrats currently enjoy a 29-to-21 edge in governorships and seem likely to pad their margin by a few.

Here are some of the elements running through the campaign politics of 1990:

Taxes: For the past decade, voters have been telling pollsters they're willing to support taxes earmarked for popular programs. Occasionally, they'll even vote that way -- as Californians did this spring when they approved a ballot initiative raising taxes to pay for freeway improvements.

But that's an exception to what remains a pretty firm rule: If you are running for office and perceived as less anti-tax than your opponent, you've got a problem.

The most recent candidate to feel the pinch of this axiom is, surprisingly, a Republican -- Illinois gubernatorial nominee Jim Edgar. He has seen his double-digit lead over Democrat Neil Hartigan shrink to just 2 points as a result of a flurry of Hartigan television ads reminding voters that Edgar supports making a temporary income tax surcharge permanent and that Hartigan opposes this. "Right now, we're net losers on the tax issue," acknowledged Sipple, Edgar's media consultant. "But we hope to turn it around by making the issue integrity."

Because the sluggish economy has created budget shortfalls in most states, positioning on possible tax increases will dominate many state races this fall. It also will be a key issue in federal races.

When Bush abandoned his no-new-taxes pledge two months ago without telling voters what new benefits they might enjoy if taxes were raised, he created a political problem for GOP Senate and House candidates. Many have parted company with their president and will stick with the "read-my-lips" formula of 1988 -- meaning that, for the first time in more than a decade, the Republican Party enters the fall campaign without a unified economic message.

The Democrats don't have one, either (a much less unusual circumstance), although some candidates, led by New York's Gov. Mario M. Cuomo, are beginning to blow the bugle of economic populism by calling for tax increases on the wealthy.

Persian Gulf: The long-term political risk for Bush is that the United States will get bogged down in the region, spending lives and money on a mission that has not been clearly defined. Are the stakes the American way of life -- as Bush said early in the confrontation -- or merely the American style of life? If things go badly in the desert, that question could haunt him in 1992.

But in the short term, Bush has won bipartisan plaudits for skillfully marshaling both military and diplomatic assets -- and in dramatically making the point that in the new post-Cold War world order, there is still one superpower left. "The president has done everything right, in my judgment," said Sen. Joseph R. Biden Jr. (D-Del.), voicing the kind of rally-round-the-flag support that Bush is likely to enjoy for the foreseeable future.

On the campaign trail, there have been only the barest murmurs of dissent. They have tended to be concentrated in the historically isolationist Midwest. In Iowa, Sen. Tom Harkin (D) has questioned the need for calling up the reserves and in Nebraska, Sen. J. James Exon (D) has raised questions about the failure to alert Americans living in Kuwait of the dangers of an Iraqi invasion; he also was quick to call for ruling out the use of force to expel Iraq from Kuwait.

Exon's opponent, former representative Hal Daub (R), has accused him of advocating surrender. Harkin's opponent, Rep. Thomas J. Tauke (R), has accused him of being "tragically misinformed or blatantly opportunistic."

Social Issues: In 1989, the biggest 1990 issue looked to be abortion. But in 1990, the S&L bonfire, the fears of a recession and now Operation Desert Shield have moved abortion off center stage.

This month's Senate confirmation hearings on the nomination of Judge David H. Souter to the Supreme Court are likely to raise the abortion issue anew, and it is sure to be a critical factor in isolated races -- especially at the gubernatorial level.

Races this fall offer every possible variety of abortion matchup -- including, in the Kansas gubernatorial race, an anti-abortion Democratic woman, state Treasurer Joan Finney, opposing an abortion-rights Republican man, Gov. Mike Hayden. But as a general rule, this is an issue that should help Democrats and female candidates -- including the record 13 women running for either governor or senator.

Redistricting: Nineteen congressional seats are scheduled to switch states in 1992 as a result of redistricting, with the losers mostly in the Northeast and Midwest, and the gainers in the Sun Belt.

The key to the redistricting battle is for one party to win both the governorship and the state legislature of the affected state so it can control the map-writing process. The Democrats made such a sweep in New Jersey in 1989. They have a shot at achieving effective control in the two biggest growth states -- California (expected to gain seven House seats) and Florida (plus four seats) -- by winning those two governorships, and the three biggest deficit states -- New York (minus three), Pennsylvania (minus two) and Illinois (minus two) by winning control of the state legislatures.

Mavericks: In Connecticut, former Republican senator Lowell P. Weicker Jr. is bidding to become the first independent in recent history to win a governorship; in Massachusetts, blunt-talking Boston University President John Silber (D) is running the classic outsider's race against the state Democratic establishment; and in Florida, former senator Lawton Chiles is trying to cleanse modern politics by winning a big state governorship without accepting a contribution of more than $100.

The theme that connects these disparate campaigns is not ideological, but disgust with politics as usual.

Ad Watching: In several states with high-profile gubernatorial races, including California, Texas and Florida, major newspapers and television stations now routinely run stories on the accuracy and fairness of all political ads that appear on television.

It is not yet clear whether this newly aggressive journalistic scrutiny has raised the level of the campaign dialogue, or simply added one more voice to the already noisy contention over truth, innuendo and false inference.

Staff researcher Bruce Brown contributed to this report.