To the casual observer, politics is hit or miss - but anyone who studies the game knows the importance of trends. Though never hard and fast, previous trends are often a decent gauge for upcoming elections. If the economy is in a recession, the incumbent president should probably start clearing out the Oval Office. If the nation is in a war, the opposite is true, and even a crook like Richard Nixon can easily be re-elected. In a midterm election, the president's party almost always loses seats in the House of Representatives. Since the Civil War, there have been only three midterm elections where the president's party managed to escape that fate - though two of those happen to be the last two midterm elections.

Anything goes when the exception becomes the norm, but don't expect the 2006 midterm elections to go the way of the last two. There were good reasons for the exceptions in 1998 and 2002 (the failed impeachment of a popular president and the Sept. 11 attacks, respectively). There are no such circumstances to link 2006 to the trend-bucking elections of 1998 and 2002.

There are many striking similarities, however, between this election and 1994 - and that would mean that the Republicans might as well start their goodbyes. In that year, after holding the House of Representatives for the better part of 60 years, the Democrats lost an astounding 54 seats in the House. Under the absolute leadership of the brash Newt Gingrich, the Republicans wrested control of the House, and they've held it ever since. That, coupled with a less dramatic but equally significant takeover of the Senate, brought on "the Republican Revolution." With a resounding victory for President Bush and solidified Republican control over both houses of Congress in 2004, the revolution seemed complete.

But in politics, all victories are temporary, and revolutions can be countered. Leading up to 1994, it was the Democrats who had a president with a low approval rating and unpopular policies. In addition, they had to deal with ethics investigations - like the one that forced House Speaker Jim Wright to resign in 1989 - and messes like the house banking scandal, exposed by the bloodthirsty "Gang of Seven" freshman Republican congressmen. The Democrats controlled both branches of Congress and the presidency, and they took the blame squarely on election day.

And now, 12 years later, it's remarkable how the tables have turned. It's the Republicans who must defend a president with abysmal approval ratings and a war that's becoming increasing unpopular, too. And if that's not enough, they must somehow mitigate the party's illicit ties to a sketchy lobbyist, Jack Abramoff, and disgraced congressmen such as Tom Delay and Mark Foley. Surely everything can't be the Republicans' fault, but with their ironclad control over both houses and the presidency, who else can voters blame?

And so voters face what certainly is the most momentous midterm election since 1994. With Republican incumbents in danger across the country, we offer our analysis of some of the races most pivotal in determining control of Congress. These are by no means the only important races - or even most of them - but space constraints allow us to include only a handful. We have broken them down by region and hope to offer you a hint of the monumental scope of this election.

We owe much of our information to the excellent interactive election guide that The New York Times has posted on its website. We encourage you to consult this or other election guides and become informed on the changes that next week's elections may bring. After all, Tuesday's results could very well determine the next decade of American politics.

Imran Syed

Midwest

Driven by a sustained economic downturn and growing disillusionment with the war in Iraq, voters in the Midwest have made their region the toughest of all for Republicans to hold onto.

Senate

Missouri: Republican incumbent Jim Talent seeks re-election against Democratic challenger Claire McCaskill in a race that many are calling the closest in the country.

Fueled by ballot initiatives that would allow stem-cell research in Missouri if approved at the federal level and a proposal to raise the minimum wage, Missouri has become a battleground over the economy and socially conservative ideals. Talent - who defends the war in Iraq, proposes a ban on embryonic stem-cell research and opposes an increase in minimum wage - is struggling to justify all three positions to the moderate rural and corporate constituents in his base. McCaskill, on the other hand, supports both proposals and has questioned the administration's war in Iraq, much to the delight of Missouri's urban population. Prediction: McCaskill prevails in the nation's tightest race.

Ohio: Two-term Republican incumbent Mike DeWine is also struggling against his Democratic challenger, Sherrod Brown.