Overreliance on rosy polls from friendly pollsters is giving Democrats a false sense of security.

With exactly one week to go before Election Day, many Democrats feel they have plenty of reasons to feel optimistic. 

It’s true: Polls show them leading on the generic congressional ballot, and some election analysts believe Democrats are well positioned to regain control of the House.

But there is a difference between being favored to win and having already won. Democrats who treat a blue wave as inevitable may be setting themselves up for an unpleasant surprise.

The latest Reuters/Ipsos poll found that registered voters preferred Democrats for Congress by 47% to 36%. 

On its surface, the poll presents an encouraging picture for liberals.

But among likely voters, including those who leaned toward one party, the margin was narrower: 53% to 44%. 

Either way, the poll suggests a difficult political environment for Republicans. Other surveys have also shown Democrats ahead, making it impossible to dismiss the picture as one bad poll.

The Cook Political Report, which assesses individual House races, now expects Democrats to win between 227 and 237 seats. That would be a comfortable improvement over the 218 needed for a majority. Cook has moved dozens of races toward Democrats as Republicans find themselves defending territory they once expected to hold more easily.

Those numbers do not, however, constitute an election result.

Polls measure the preferences of the people who respond, and pollsters work to make those respondents resemble the larger electorate. But the electorate on Election Day is not everyone who is registered, or even everyone who tells a pollster they intend to vote. 

It is the smaller group of people who actually cast ballots.

Predicting that group is especially difficult in a midterm. Some voters participate in every election. Others turn out only when they feel strongly about a candidate or issue. Some are angry enough to tell a pollster about it but not motivated enough to visit a polling place. Others say little and vote anyway.

Republicans have ample reason to believe the polls may underestimate their support. 

CNN’s Harry Enten has pointed out that early-October generic-ballot polling underestimated Republicans in 14 of the past 16 elections, by an average of about four points. That does not mean every late-October poll is wrong by four points, or that the error will necessarily favor Republicans this year. It does mean a Democratic lead can look different once the votes are counted.

Speaker Mike Johnson argues that Republican enthusiasm is being missed because many conservative voters do not trust pollsters or want to participate in surveys. That is possible. It is also possible that Democrats really are more energized: the Reuters/Ipsos survey found a considerable Democratic advantage in the share of voters who described themselves as very enthusiastic about voting.

We will not know which assessment better reflects the electorate until the votes are cast.

There is another complication. A voter can be dissatisfied with President Trump and still vote Republican for Congress.

Gas prices are high. Many Americans are frustrated with the cost of living, and some are unhappy about the war in Iran. Those concerns are real, and Republicans would be foolish to ignore them. But a voter who is unhappy about gasoline prices may also be skeptical that Democrats would bring them down.

That same voter may prefer Republican policies on the border, taxes or energy. Or he may worry that a Democratic House would spend the next two years consumed by investigations rather than addressing the problems families face.

Johnson calls this a “contrast election.” He wants voters to consider not only their frustrations with the current administration but also what they would get if they handed Congress to the other party. Whether that argument succeeds will depend on the candidates and the voters in each district.

Democrats also face questions about the direction of their party. The Democratic Socialists of America has advanced proposals including the eventual abolition of police and prisons. Those are not the positions of every Democratic candidate, and the DSA is not the Democratic Party. But voters are entitled to ask individual candidates where they stand and how much influence the party’s most progressive members would have in a new majority.

House elections are decided district by district, not by a national poll.

A popular incumbent can survive a difficult year for her party. A strong challenger can win in territory that usually favors the opposition. Redistricting can create opportunities, but it can also produce districts that become vulnerable when the political environment shifts.

Republicans are defending seats they hoped would be safer. Democrats, meanwhile, must turn promising poll numbers into votes in the right places. Even a national Democratic advantage does not tell us precisely how large their majority would be — or guarantee that they will have one.

A blue wave remains a real possibility. The warning signs for Republicans are too numerous to pretend otherwise. But elections have a way of complicating confident predictions. A few points of polling error, a late change in enthusiasm or a cluster of close races breaking the other way could produce a far narrower result than Democrats expect.

For now, the polls tell us who appears to be ahead. Only the voters can tell us who wins.

(Contributing writer, Brooke Bell)