With just four days left until the midterm elections, the latest opinion polls are indicating that the Democrats will take control of the House of Representatives, and the Republican Party will retain its narrow majority in the Senate—and possibly even expand it. However, many races across the country appear to be really close, which adds a good deal of uncertainty to the outcome.

Broadly speaking, the latest polls are suggesting that the Democrats are doing well enough in the suburbs and exurbs to pick up the twenty-three G.O.P.-held seats in the House they need to flip for a majority, and possibly a dozen or two more. In places like northern New Jersey, southeastern Pennsylvania, and Orange County, California, the suburban backlash against Trump seems to be holding up. But the Republican vote appears to have strengthened in areas of the country that Trump carried easily in 2016. This is shoring up G.O.P. incumbents like Ted Cruz, in Texas, and Cindy Hyde-Smith, in Mississippi, and endangering red-state Democratic incumbents, such as Heidi Heitkamp, in North Dakota, and Claire McCaskill, in Missouri.

Further Reading New Yorker writers on the 2018 midterm elections.

These trends point to a balance of power on Capitol Hill for the next couple of years, and the likelihood of legislative gridlock. However, there is always a chance that the opinion surveys are giving a misleading impression. On the one hand, the pollsters could be underestimating the Democratic vote, particularly among young people and minorities, who tend to have low turnout rates in midterm elections. If a powerful anti-Trump surge materializes in these groups, the much discussed “blue wave” could still drown the G.O.P. Conversely, the pollsters could be underestimating Donald Trump’s ability to turn out voters attracted to his divisive message, as they did in 2016.

Partly based on my memories of the midterm elections of 1994, 2006, and 2010, when the parties of Presidents with low approval ratings suffered big losses, I think the first type of polling error is more likely this year. But if 2016 taught us anything, it’s that we should be cautious in making predictions, particularly when so many districts are being hotly contested.

How many? The Cook Report reckons that seventy-three seats are still competitive, which is a pretty large number in this era of systematic gerrymandering. Of these seats, it rates twenty-nine, almost all G.O.P.-held, as “toss-ups.” (The others it rates “lean Democrat” or “lean Republican.”) Using FiveThirtyEight’s immensely useful polling database, I looked up the latest poll results in the twenty-nine most tightly contested races and found that in all but one of them the margin between the two candidates is four percentage points or less, which means it is well within the statistical margin of error. These are genuinely close races.

Still, the balance of the polling data favors Team Blue. According to a new poll of battleground seats from the Washington Post and Schar College, which was released on Thursday, voters in battleground districts favor the Democratic Party over the Republican Party by four percentage points. Although that is a slim margin, it represents a big turnaround from 2016, when voters in these districts favored Republicans by fifteen percentage points.

The new poll also confirmed that the Democratic Party now has a huge advantage over Republicans among nonwhites (sixty-six per cent to twenty-six per cent); people under the age of forty (fifty-eight per cent to thirty-seven per cent); and white college-educated women (sixty-two per cent to thirty-six per cent). On Tuesday, the latter group can probably be relied upon to vote in large numbers. The results of the election, though, may well hinge on turnout in the first two groups. Obviously, we won’t know until after the election. But on Wednesday, Tom Perez, the head of the Democratic National Committee, cited the number of young people who had voted early in Georgia as a good sign for his party.

Another key variable is the voting pattern of self-identified Independents, a group that makes up at least a third of the electorate. One clue here comes from Orange County, once a bastion of suburban Republicanism, where Democrats are hoping to pick up four seats. The polls continue to point to tight races in at least three of these races; they also show non-aligned voters breaking to the Democrats. In “every poll that we have done in Orange County, the Democrats command a very, very, very wide lead with NVP”—non-aligned—“voters,” Nate Cohn, a member of the Times’ Upshot team, which, with the Siena College Research Institute, has been diligently carrying out polls all across the country, wrote in an article published on Thursday. “And that’s important for Democrats because in all these districts the Republicans are likely to have a registration advantage.”

A similar phenomenon is visible in some Republican-held districts on the East Coast. Take New Jersey’s Eleventh District, where the Democrat Mikie Sherrill is going up against the Republican Jay Webber in a closely watched race. Pollsters at Monmouth University recently found that Sherrill had a narrow lead, of four percentage points. But the polls also found that fifty-eight per cent of Independents favored Sherrill, and just twenty-eight per cent favored Webber, whom Trump has endorsed.

In the fight for the Senate, there are also a number of tight races. The Democrats need to make a net gain of two seats to take control. At this stage, their best bets for picking off Republicans appear to be in Arizona, where the Democratic congresswoman Kyrsten Sinema is battling the G.O.P. congresswoman Martha McSally for an open seat, and in Nevada, where another Democratic congresswoman, Jacky Rosen, is challenging the Republican incumbent, Dean Heller. On Thursday afternoon, the Real Clear Politics poll average showed Sinema leading McNally by 0.7 percentage points, and Rosen trailing Heller by two percentage points. Both of these races are far too close to call.

Democrats also retain hopes of pulling off upset wins in Tennessee, where the former governor Phil Bredesen is taking on the Republican congresswoman Marsha Blackburn for an open seat, and in Texas, where the challenger Beto O’Rourke has put a fright into the incumbent, Cruz. However, recent polls have shown Blackburn and Cruz establishing seemingly solid leads: 6.8 percentage points and 6.5 points, respectively, according to the Real Clear Politics average.

Meanwhile, the latest polls have not indicated much, if any, improvement for the embattled Heitkamp, or for Missouri’s McCaskill, another Democratic incumbent in a deep-red Trump state who is in trouble. The Real Clear Politics poll average shows Heitkamp trailing the Republican congressman Kevin Cramer by 11.4 percentage points, and McCaskill two points behind the Trump enthusiast Josh Hawley. At least three other Democratic incumbents are also engaged in extremely tight races: Indiana’s Joe Donnelly, who is going up against the Republican businessman Mike Braun; Florida’s Bill Nelson, who is taking on the G.O.P. governor, Rick Scott; and Montana’s Jon Tester, whose opponent is Matt Rosendale, a Republican state official. The Real Clear Politics poll average shows Donnelly leading by 0.8 percentage points, Nelson by 2.3 percentage points, and Tester by 4.5 percentage points.

To repeat, the poll numbers could turn out to be wrong. To cite one example, some of O’Rourke’s supporters have long argued that pollsters are greatly underestimating how many young Texans and Latino Texans will turn out for him. That could conceivably be true. But for the Democrats to win control of the upper chamber, it would probably have to be true in most or all of the other key Senate races as well. That’s why the polls-based forecasting models, such as the one operated by FiveThirtyEight, are now showing the Republicans heavily favored to maintain control.

Of course, what really matters are the actual votes, not the polls or the predictions. And with so many contests going down to the wire, we are in for a nerve-jangling few days.