Likely Gains and Losses Democrats hope to hold Republican House gains to around 30 seats, but Republicans foresee gains from the mid-40s all the way up to 100. Impartial observers see the Republicans adding perhaps 45-65 seats. They could have 230 or more seats in the new Congress, well over the 218 needed for control.

By contrast, Republcan leaders are not expecting to win the Senate. They can see eight or nine seats coming their way, but the 10th is elusive.

Republicans expect to sweep gubernatorial battles in many states that are usually Democratic, including Illinois, Iowa, Michigan, Ohio, Pennsylvania, and Wisconsin. They dream of an upset in California as well.

Along with the new governorships will come maybe 500 additional Republican state legislators and control of a dozen state legislative chambers. Results predicted Races to watch

Everything we have seen in the current campaign suggests that 2010 will be very unlike 2008.

Democrats celebrated a new president and large majorities in both houses of Congress two years ago, but Republicans are poised to do the celebrating this time around.

On the other hand, election nights are full of surprises, and they never turn out exactly the way prognosticators have predicted.

What should we be looking for on election night?

States close their polls at varying times, and the 48 continental states stretch over four time zones. The first two states to report will be Kentucky and Indiana.

First results

If the Republican Tea Party nominee for US Senate, Rand Paul, is not winning in Kentucky, any lingering Republican hopes for Senate control will be dashed. A strongly anti-Obama state, Kentucky should vote Republican this year, but Paul has been a controversial nominee.

Democrat Jack Conway, the state's attorney general, is his opponent.

In Indiana, Republican Dan Coats is expected to defeat Democrat Brad Ellsworth for the Senate seat of retiring democratic Senator Evan Bayh. This is a must-win for the Republicans.

Look also to see whether any incumbent Democratic House members are losing re-election in these two states, as this could be a sign of things to come elsewhere. Among the most endangered are Ben Chandler in Kentucky's sixth District and Baron Hill in Indiana's ninth District.

East coast

Within a couple of hours - not long after midnight GMT (2000 on the East Coast of the US - tabulations will begin to cascade from states on the eastern seaboard.

Image caption Tea Party Senate nominee Marco Rubio is leading polls in Florida

If Democrats are having an unexpectedly good night, then Democrat Joe Sestak will defeat Republican Pat Toomey for the Pennsylvania Senate seat of retiring Democratic Senator Arlen Specter, and Democrat Alex Sink will capture the fiercely contested Florida governor's race over Republican Rick Scott, becoming the first woman chief executive of the sunshine state.

Also look to see how many Democratic incumbents are falling to Republican challengers in these large, critical battlegrounds.

Hard-pressed Democratic congressmen include Chris Carney in Pennsylvania's 10th District and Allen Boyd in Florida's second District.

One other Florida race worth watching: Republican Marco Rubio, a Tea Party favourite, is well ahead in the polls to take a US Senate seat.

If either Independent Charlie Crist or Democrat Kendrick Meek, who is strongly backed by former President Clinton, come close to Rubio, Republican brows will furrow.

Midwest

Once we arrive in this region, most eyes will be on Ohio and Illinois.

In Ohio, a key swing state that decided the 2004 presidential election, Republicans are very likely to win a Senate seat with former Bush White House official Rob Portman defeating the Democratic Lieutenant Governor Lee Fisher.

The full picture may not be known for days - some close races will go to recount, and results from Washington state and Alaska can take a week or more

But the governorship race is close.

President Obama wants a friend in the statehouse as he seeks a second term in 2012, so he has made the re-election of democratic Governor Ted Strickland a high priority.

Still, the GOP's John Kasich, a former congressman, is thought to be slightly ahead.

Illinois matters because it is Mr Obama's home state, and it would be a great embarrassment to him if either the governorship or the Senate seat he once occupied were lost to the Republicans.

Democratic Governor Pat Quinn, the successor to the resigned scandal-drenched Rod Blagojevich, may well lose to Republican challenger Bill Brady.

Still, Democrats are hoping they can rescue Senate nominee Alexi Giannoulias, a longstanding Obama friend. The Republican candidate, Congressman Mark Kirk, has maintained a tiny edge in pre-election polling.

Further west

Image caption A win for Harry Reid would delight Democrats in the face of other losses

The West is full of revealing battles and tight contests.

None matters more than the Nevada Senate match-up between Democratic Senate Majority Leader Harry Reid and Tea Party Republican Sharron Angle.

Mr Obama carried Nevada and desperately wants to avoid a defeat for Mr Reid, the chief patron of much of his legislation.

Yet here again, despite Ms Angle's many gaffes and very conservative positions, Reid has no better than a 50-50 chance of reelection.

Reid's defeat would colour the night Red. But his victory would delight Democrats even in the face of other losses.

Senate races in California, Colorado, and Washington may well determine control of the Senate.

The three Democratic incumbents, Barbara Boxer, Michael Bennet, and Patty Murray, are all endangered to one degree or another.

If all win, Republican hopes for a Senate takeover are gone.

If one or two lose, and Republicans win the other competitive seats around the nation in which they are thought to be competitive, then the Republicans could eke out the narrowest possible majority of 51 in January's Senate.

It is important to note that trends can change across this continental country as the night wears on.

Republican victories in the East and Midwest may or may not be repeated in the Rocky Mountain or Pacific states. A Republican red "wave" may sweep unevenly across the nation, and there could be Democratic blue "sandbars" that don't get wet, such as New York and California.

The full picture may not be known for days. Some close races will go to recount, and places such as Washington state (full mail balloting) and Alaska (delayed tallies from the outlying "bush" country) can take a week or more to post convincing proportions of the votes.

Larry Sabato is editor of Sabato's Crystal Ball and author of the forthcoming book, Pendulum Swing, the story of the 2010 mid-term elections