With the midterm congressional elections only a few days away, Republicans face a toxic political environment, fouled in large part by growing bitterness about the handling of the war in Iraq. By almost every measure, voter discontent is deeper than it was in 1994, when Democrats lost nine seats in the Senate and 54 in the House.

Surely the Republican Party is facing an electoral drubbing. And yet President George W. Bush and his chief political strategist, Karl Rove, baldly assert that Republicans will retain control of Congress. Majorities in Congress aren't formed by the national zeitgeist, as Rove cheerfully points out. They are built one race at a time — and, in dozens of close contests this fall, the outcome will be determined largely by one often-overlooked minority group: the mostly white, mostly conservative voters who live in America's small towns.

Residents of rural areas make up only a fifth of the country's population. That's a little less than African-Americans and Hispanics combined. But, unlike voters in those minority groups, small-town whites are often kingmakers in national politics. In 2004 they voted for Bush by nearly a 20-point margin. Newspapers ran headlines that baffled their urban readers: "Rural Values Proved Pivotal," "Conservatives in Rural Ohio Big Key in Bush Victory" and "G.O.P. Won With Accent on Rural and Traditional."

This year those same right-leaning small towns make up a major voting bloc in a half-dozen make-or-break Senate races, including those in Missouri, Montana, Tennessee and Virginia. They also dominate battleground House districts throughout the country, from Idaho to northern New York. If rural America embraces Republicans with the same fervor it did two years ago, Democrats will almost certainly be denied a majority in the Senate and may fall short in the House. In part the electoral importance of small towns reflects a profound pro-rural bias hardwired into our political system. The Constitution grants two Senate seats to each state, regardless of its population. As a consequence a majority of senators are elected by voters in 26 sparsely settled states that together contain less than 18 percent of the country's population.

Today the Republican Party holds an 11-seat Senate majority, but Republican senators represent 4.5 million fewer people than their Democratic colleagues, who tend to come from urban states such as California, Illinois and New York. In the 2004 elections Democratic candidates for the Senate captured nearly 10 percent more votes than Republicans nationwide, thanks to landslide support among urbanites, yet the Republicans still managed to gain four seats, due to victories in rural states such as South Carolina and South Dakota.

A similarly skewed outcome is possible this year. Democrats are widely expected to gain seats in Ohio and Pennsylvania. If they do, Democratic senators will represent some 10 million to 20 million more Americans nationwide than will Republican senators. But if rural voters in Missouri, Tennessee and Virginia swing conservative, the Republicans will cling to the narrow majority Rove has promised.

Republican leaders in the House, including Speaker Dennis Hastert (R.-Ill.), have worked to create a similar small-town bias. They redrew district lines to systematically fracture the voting power of urban and suburban neighborhoods in half a dozen states, including Florida, Ohio and Texas.

In 2003 The Washington Post published an analysis of this scheme by a Texas political operative, Joby Fortson: "This has a real national impact," Fortson wrote, "that should assure that Republicans keep the House no matter the national mood." Naturally the Republicans' rural strategy works only if small towns vote Republican by wide margins. This year that might not happen. A bipartisan poll conducted last week for the Center for Rural Strategies found that the loyalty of rural voters has been deeply shaken by Iraq and by the Mark Foley page scandal.

Since September, rural voters have moved from a four-point advantage for Republicans to a four-point advantage for Democrats in Senate races, the Rural Strategies report concluded, and from evenly divided to a 13-point advantage for the Democrats in House races. Worse yet for Republicans, a growing number of Democrats have awakened to the fact that small towns matter.

Nationwide Democrats are fielding centrist candidates who can compete in rural districts such as Idaho's First, Indiana's Second and New York's 24th. If Democrats succeed in increasing their rural vote, they could decisively sweep Republicans from power. But, as the Center for Rural Strategies has pointed out, most of these races will be decided by razor-thin margins. And the Republicans are working feverishly to mollify and to re-energize their rural base with talk about same-sex marriage, abortion, gun rights, public Christianity, terrorism and immigration, all issues that play brilliantly in small towns. The Republican National Committee has cranked up its sophisticated get-out-the-vote machine, combining phone and mail prompts, pastor-and-pulpit networks, conservative talk radio and door-to-door canvassing.

On Election Day, millions of urban Democrats will go to the polls expecting victory and dramatic change in Washington. But beware: Rove's sunny forecast isn't mere spin. He and his party are counting on small towns to send a very different message, and to give the Republicans two more years to get it right.

Brian Mann, author of "Welcome to the Homeland: A Journey to the Rural Heart of America's Conservative Revolution," is a Saranac Lake, N.Y.-based reporter for North Country Public Radio. He wrote this commentary for The New York Times.