It’s Labor Day Weekend, and that means more than parades, barbecues, ball games, and the last swims of the season. It is also the traditional beginning of the political season, and this year — with a war in Iraq and uncertainty in the economy — the stakes are higher than usual: nothing less than control of the Congress in the last two years of President Bush’s administration.

Often midterm congressional and gubernatorial elections have no theme. Political professionals are fond of regarding them as 500 separate races, each with its own character, its own rhythm, its own pressure points. Not this time. President Bush has low approval ratings, the Republicans’ margins in the two houses are blade-of-grass slim, and it is not too much to suggest that the nature of both parties is up for grabs, determining how conservative the Republicans want to be and how liberal the Democrats want to be.

Indeed, not since 1982 (when the Democrats transformed the first elections of the Reagan era into a referendum on the future of Social Security) or perhaps 1994 (when the Republicans transformed the first elections of the Clinton era into a referendum on the Democrats’ 40 years of control of the House) has there been so national a cast to off-year elections. Here are the themes that are being contested.

* War in Iraq. Generally speaking, the Republicans are standing by their man in the White House, and the Democrats are assailing him. But the variations within the Democratic Party are telling. Some Democratic candidates (Rep. Sherrod Brown, who is challenging Senator DeWine of Ohio, and Ned Lamont, who defeated Senator Lieberman in Connecticut for the Democratic senatorial nomination) are urging withdrawal from Iraq at a certain date, while others (Senator Cantwell of Washington) are arguing that as odious as the conflict is, withdrawing could be even worse for American foreign policy around the world.

Some (former state attorney general Sheldon Whitehouse, the favorite in the Rhode Island Democratic senatorial primary next Tuesday) are arguing that the war was flawed from the beginning, and others (Senator Clinton) are defending their support for the war based on the information they had at the time. And some challengers (Amy Klobuchar, the Democratic candidate in Minnesota, and state auditor Claire McCaskill, the Democrat who is taking on Senator Talent of Missouri) are seeking safe ground by calling for a plan to get out — and a means to do that.

The midterm elections of 1970, contentious contests at the heart of the Vietnam conflict, were far more clear-cut than these contests 36 years later. As a result, even a Democratic takeover of one of the houses of Congress will be very difficult to interpret and will likely express generalized public impatience with the war rather than a discrete policy alternative.

* The economy. The financial markets have been unusually roiled this year, with large losses followed by large gains on Wall Street, perhaps reflecting the unsettled nature of economic prospects. This summer’s reports that real wages for workers have decreased since 2003 underline Republican vulnerability in a period where GOP lawmakers have been portrayed as ever-cozier with corporate leaders, whose wages, pointedly, have risen substantially. Whether the Democrats can transform that into a political issue is one of the major uncertainties of 2006, but candidates like Mr. Brown in Ohio are trying.

Often the issue takes the form of attacking the profits of Big Oil, and both Rep. Harold Ford, a Democrat seeking a Senate seat from Tennessee, and Rep. Mark Kennedy, a Republican seeking a Senate seat from Minnesota, are emphasizing that theme. Candidates in both parties in both the gubernatorial and senatorial races in Michigan, where business conditions are weak, are stressing the economy, with the Republicans blaming the Democratic incumbents, Governor Jennifer Granholm and Senator Stabenow, and the two Democrats in turn blaming President Bush.

* The Democrats. Frustrated by their powerlessness in the Bush era, Democrats are flailing right now, struggling to regain power but also to regain their footing. Within the party rages a furious battle, which might be distilled into a struggle over whether it is safe to use the word “liberal” again. This battle played itself out most dramatically in the bitter primary struggle last month in Connecticut between Senator Lieberman and Mr. Lamont.

Though Senator Clinton will sail to renomination and re-election in New York, some Democrats remain uneasy about her support for the war — not enough to allow Jonathan Tasini to make even a small dent in the Clinton armor in next Tuesday’s primary, but enough to provide an important obstacle should she decide to pursue the Democratic presidential nomination in 2008.

These strains are well beneath the Democratic waterline this fall, but are not insignificant; they may explain, for example, Senator Clinton’s announcement last Friday that she would sponsor a fund-raiser and campaign beside Mr. Lamont, the insurgent who has emerged as the symbol of an Internet-based, left-leaning Democratic rebellion. These strains, moreover, are perhaps the most important elements of this fall’s contests as the party positions itself for an election where it will not face an incumbent and thus must define itself rather than respond to an incumbent’s record.

* The Republicans. There are strains here as well, with tests for party members who are both conservatives and moderates. If, for example, Senator Santorum of Pennsylvania is defeated, it will be a blow to conservatives, who have embraced Mr. Santorum as the youthful face of the movement. At the same time, if Senator Chafee of Rhode Island is defeated in next week’s primary by Mayor Steve Laffey of Cranston, or in November by Mr. Whitehouse, then it will be a defeat for New England moderates. Such are the uncertainties, and the potential story lines, of an election that already has been nationalized.