We’re gearing up for what is certain to be a very exciting and — given that there are a number of crucial races on the West Coast, particularly for the Senate — very long Election Night.

Among other things, we’re hoping to be able to update our House and Senate takeover projections as the night progresses. While we almost certainly won’t be updating our forecasts for individual seats, and we definitely won’t be “calling” any races until The New York Times does, we do hope to provide some forecast of the overall number of seats that Republicans are most likely to win in each chamber, and their probability of taking over both the House and the Senate. These top-level projections would be updated a couple of times an hour as the returns roll in.

In the process of preparing our model to do this, I’ve started to home in on the seats that are likely to tell us the most about the disposition of the House as Election Night progresses.

In particular, what I’ve done is to take all 435 House seats and sort them in order of the margin we project in each one — from the Republican Ron Paul’s district, the Texas 14th, which we expect him to win by about 65 points, to the Democrat José E. Serrano’s New York 16th in the Bronx, where he should be re-elected by 70 points or so.

The outcomes of lopsided races like Mr. Paul’s and Mr. Serrano’s aren’t likely to be terribly exciting to many people other than Mr. Paul, Mr. Serrano and their immediate families. But a tremendous number of House seats are competitive this year.

In a series of charts below, I’m going to list what our forecasting model considers to be the roughly 150 most competitive House races, as of Saturday afternoon. This obviously casts a very wide net; it’s basically every race in which we expect the two candidates to finish within 20 points of each other.

The charts are organized by the time that we should expect to see returns starting to trickle in from each state. There are few judgment calls involved here, based on the past experiences of The Times’s data team, as some states have multiple poll closing times. As veteran election-watchers know, Indiana and Kentucky, where most polls close at 6 p.m. Eastern time, should be the first states to begin reporting results. Alaska, meanwhile, won’t start releasing vote totals until 1 a.m. Eastern, after people in the Aleutian Islands have had their chance to vote.

The House seats are further divided into three columns. The seats that reflect the G.O.P.’s path of least resistance to taking over the House are in the leftmost column. If the Republicans won exactly the seats in the left-hand column, but no others, they would gain a net of 39 seats from Democrats, and control the House, 218-217. This basically involves those seats where the Republican candidate is favored by 3 or more points by our model.

Seats where the G.O.P. could begin to build on their majority are in the middle column. These are seats that Republicans they are favored to win by our model, but by fewer than 3 points. If Republicans won each of these seats, their gains would total a net of 59.

(A technical note: our simulations have the Republicans picking up an average of 53-54 seats, rather than 59. This is because the forecasts are somewhat asymmetric: there are 14 seats in which we have the Republican favored by 0 to 2 points, versus 6 seats like this for Democrats. Our official forecast looks at these races probabilistically — that is, if our model has the Republican projected to win by 0.01 points, his winning chances are 50 percent and some very small fraction. We don’t “call” the race for him. But if we do allocate all of the toss-up seats to one or another party, no matter how trivial its lead, we’re showing Republicans as favorites to win a net of 59 seats from Democrats.)

Finally, the seats in the far-right column reflect the tsunami possibility: those that Republicans would need to win to achieve a gain of 60 seats or more.

Each competitive race is represented by a little box that contains several key pieces of information. For the most part, this should be pretty self-explanatory:

The one thing that I’d like to draw your attention to is the statistic in parenthesis in the lower left-hand corner of the box: this is what we call the magic number. What this statistic indicates is how many seats we’d expect the Republicans to gain on the Democrats over all if they won this particular seat and all seats in which we have them favored by a larger margin.

In this particular example, for instance — the New York 19th congressional district, in which we have the Republican Nan Hayworth favored by 3 points — the magic number is 43. That means that if Republicans won this seat — and all other seats in which they were favored by more than 3 points, but none of the seats in which we had them favored by fewer than 3 points — they would finish with a gain of 43 seats on Democrats over all on the night. Another way to look at the magic number is that it’s the number of seats we’d expect Republicans to win nationwide if they won this particular district by exactly 1 vote (and we had no information about what had taken place in any other district).

Occasionally, the magic number will be negative; these are seats that, if the G.O.P. lost them, would imply that it were actually going to lose seats in the House overall.

What you should be looking for is whether Republicans are consistently winning seats with magic numbers in the 60s, 70s, 80s or higher. If so, they could be in for a very big night. Conversely, if Democrats are holding onto seats with magic numbers in the teens, 20s, or 30s, that means they are overperforming their forecasts and could hold the House.

By consistently, by the way, I do mean consistently: individual districts are fairly hard to forecast, and so Republicans almost certainly will pick off a few seats with very high magic numbers, and Democrats will almost certainly hold on to some with very low ones, regardless of what is happening elsewhere in the country.

You should also be watching the margin of victory in each district, particularly for districts in which enough of the vote has been counted that The Times has called the race. Our forecasts are calibrated to an overall Republican gain of between 50 and 60 seats. If they’re consistently winning their races by a larger margin than our model expects, that means their gains are likely to be somewhere beyond 60 seats. If they’re underachieving their margins, on the other hand, it may be below 50 seats, and Democrats might hold the House.

One reason we do try to be so precise with our forecasts — projecting a margin of victory in each race rather than putting them into broad categories like “toss-up” and “lean Republican” — is exactly so that it can serve this benchmarking function. It’s not that we know exactly what is going to happen on Tuesday; quite to the contrary, we think other forecasters are being incautious in not acknowledging the degree of uncertainty inherent to forecasting this House election.

But we can show you the blood, guts and entrails of roughly what a Republican gain of about 55 seats would look like, if that turns out to be the number: it would look something like this.

All right, that’s enough buildup. Let’s show you what to look for beginning at 6 p.m., when we’ll begin to see the first results from Indiana and Kentucky.

Baron Hill’s seat, the Indiana 9th, has long been one of the most competitive in the country. I don’t think you should get too swept up in the results of any one particular congressional district — not when there are 435 of them in every corner of the country. But Mr. Hill, a middle-of-the-road Democrat who ordinarily performs strongly in his fairly rural, somewhat Republican-leaning district, but who voted for the health care bill and the stimulus, is in a position that is fairly typical for Democratic incumbents around the country this year. Also, the district has a magic number of 41, which means that it’s right at the cusp of what Republicans would need to take over the House. If they fail to win it, that could be the first sign that they’re liable to do a hair worse than expected. If they win it by a margin in the high single digits or the double digits, however, it could suggest that a lot of Democratic incumbents, many of whom are less skilled than Mr. Hill at understanding how to run a strong campaign in their districts, are going to be in trouble.

Joe Donnelly, in the Indiana 2nd district, is one Democrat whose polls have held up fairly well in spite of the Republican wave. Our model has him favored by just 2 points, however, and if he were to lose, that would be a good early sign for Republicans.

Indiana’s 8th district, vacated by Brad Ellsworth, is very likely to be a Republican pickup. If they’re having trouble winning it, that’s a reasonably bad sign for them.

Indiana’s 7th and 3rd congressional districts are not likely to be especially competitive. If these races wind up within the single digits, something really weird might be afoot.

I’d be a little bit more cautious about reading too much into the two Kentucky districts on our chart, the 6th and the 3rd, just because Kentucky is a fairly idiosyncratic state to begin with, and both the polling and the Senate race have been strange there. Still, John Yarmuth’s 3rd district, which encompasses Louisville, reflects a strong potential upside case for the G.O.P. if they were to win it.

At 7 p.m. polls will close in most of Florida, Georgia, South Carolina, Vermont and Virginia.

Republicans are favored to knock off quite a few Democratic incumbents in these states, several of whom are high-profile, like Alan Grayson in the Florida 8th, John Spratt in the South Carolina 5th and Tom Perriello in the Virginia 5th. Several of these likely Republican wins could very easily be in the double-digits. The point is, however, that these are fairly low-hanging fruit for the G.O.P.: if they can’t knock off these incumbents, they’re probably going to miss on a lot of easier targets in other parts of the country. In that sense, there’s more downside than upside for Republicans in these states.

Seats like the Florida 22nd, the Georgia 2nd, the Virginia 11th and the Virginia 9th, on the other hand, are all races that we project to finish within a couple of points. If Republicans win at least 3 of these 4, that’s a pretty good sign for them.

Half an hour later, polls close in Ohio, North Carolina and West Virginia.

These states, on the other hand, involve a little more in the way of upside for the G.O.P. There are a few districts — like the Ohio 16th, 15th and 1st — that Republicans probably ought to win. There are others, like Bob Ethridge’s North Carolina 2nd district, where the polling has broken their way of late. But were they able to defeat Heath Shuler in the North Carolina 11th district, or Nick Rahall in the West Virginia 3rd — districts where the polls have generally favored the Democrats — that could suggest a rather formidable wave.

And if they were to seriously challenge David Price in the North Carolina 4th or Dennis Kucinich in the Ohio 10th, in what are ordinarily quite Democratic-leaning areas and which have magic numbers in the triple-digits, it would be hard to set an upper bound on their gains. (Even though we have Mr. Kucinich projected to win by a larger margin than Mr. Price, I’d think a loss by Mr. Price would be more troubling to Democrats, since Mr. Kucinich is more idiosyncratic and his district is more likely to behave idiosyncratically.)

Polls close at 8 p.m. Eastern in 19 states:

The bad news is that this is the point in the night at which we’re going to be quite overwhelmed with data and it may be tougher to keep an eye on the big picture. The good news is that we’re obviously going to have quite a diverse set of districts to look at by this point. If Democrats or Republicans are underperforming expectations, they will no longer be able to attribute it to regional variations or fluky circumstances.

I’d recommend focusing, then, on the seats in the middle column of the chart, or those toward the top of the left and right-hand columns. For instance, districts like the Alabama 2nd, the South Dakota at-large district, the Pennsylvania 11th, the Illinois 14th, the Texas 23rd, the Missouri 4th, the New Hampshire 2nd and the Massachusetts 10th, formulate a rather balanced portfolio of competitive races: some urban, some rural, some incumbents, some open seats. If one or another party is winning a clear majority of these districts, you’ll have a very good idea of how the night is going.

The 8 p.m. hour also features the first 2 of 4 seats that Democrats are favored to pick up from Republicans, those being the Illinois 10th and the Delaware at-large district.

Arkansas closes its polls at 8:30 p.m.:

There’s not a whole heck of a lot to look at here. The Republicans are clear favorites to pick up the open seats formerly held by Democrats in the Arkansas 1st and 2nd districts. If they fail to do so, that’s news. Likewise, it’s news if Mike Ross loses his seat in the Arkansas 4th district, or comes very close to doing so.

Nine states, including New York, close their polls at 9 p.m. Eastern:

If the data up to this point are ambiguous, we should get some relatively clear indications here. On the one hand, we have some seats, like Betsy Markey’s in the Colorado 4th, or the open seat in the New York 29th (vacated by Eric Massa) that Republicans really need to win, and some others, like the New Mexico 2nd, that also figure to go if Republicans are bound to win the House. But there are others that represent pure upside for them, like Jim Oberstar’s Minnesota 8th district, or the Rhode Island 1st, where the polling has broken toward Republicans in recent days.

There are also a ton of competitive races in New York State, many of which are fascinating on their own merits. But I’d tend to be a little cautious about reading too much into these in terms of their national implications, because of the Republicans are very weak at the top of the ticket here in New York (particularly their nominee for governor, Carl P. Paladino), and that could have some unpredictable effects.

Seven states — mostly in the Mountain West, but also Iowa, which leaves its polls open late — will start counting their votes at 10 p.m.

If Democrats are performing a little better than expected at this stage, and seem to have a chance to hold onto the House, then keeping seats like the Arizona 5th, the Arizona 8th, the Nevada 3rd and the Idaho 1st could be critical to that effort. (The lightly populated northern part of Idaho 1st is in the Pacific time zone, and polls there close at 11 p.m. Eastern, an hour later than the rest of the district.) If, on the other hand, the G.O.P. is looking to run up its numbers, then Raul M. Grijalva’s Arizona 7th, and several districts in Iowa where Democrats are favored, but not comfortably so, are places where they could add to their head count.

California, Oregon and Washington will start to report results at 11 p.m., but these states could be a little bit of a tease, as I’ll explain in a moment.

An interesting set of races here. But tracking the results could be frustrating, since Washington and Oregon vote almost entirely by mail, and many Californians do as well. On the one hand, we may see a lot of votes reported right at the outset, as these states may reveal the results of mail ballots sent in well in advance of Tuesday. On the other hand, some of their ballots will quite literally still be in the mail on Tuesday evening. Washington and California in particular are notorious for taking until several days after the election to count all of their votes. I’d tend to rely a lot on the guidance of the Associated Press and The Times in determining when it is safe to come to conclusions about these races.

Otherwise, like the rest of the country, this is a place where both parties have some upside. If the Democrats are having a much better than expected evening, there are a couple of Republican incumbents — Dan Lungren in the California 3rd, Dave Reichert in the Washington 8th — that they have some hope of beating. But ordinarily-safe Democratic incumbents like Jim Costa in the California 20th and Loretta Sanchez in the California 47th could fall if Republicans are having their way.

After midnight, Alaska and Hawaii will let it all hang out.

Only one of the three congressional districts between these states, however — the Hawaii 1st, where Colleen Hanabusa has seen somewhat improved polling against Republican incumbent Charles Djou in recent days — looks to be at all competitive.

If the major news organizations are comfortable calling the House for Republicans by midnight, then they are probably headed for gains well in excess of 39 seats. Considering the number of races that appear to be highly competitive and where virtually all of the ballots will have to be counted before a winner is declared, a few others where we’ll be awaiting the results of mail ballots, a couple of others that could be subject to runoffs, and the one or two districts where there might be some kind of irregularity, a “clean” count of plus-39 by midnight could mean that Republicans will eventually finish with gains of 50 seats or more.

We hope you’ll join us on Tuesday evening.