For many in the political community, the generic ballot test, a standardized question in most national polls, has become the electoral equivalent of the canary in the coal mine — a harbinger of things to come in November. It’s an important tool to gauge how each party is doing at any given point in time, as it gives us a fairly good, though not infallible, idea of which party is heading to the polls with the wind at its back.

This election cycle, there has been even more focus than usual on the generic ballot, with months of grim numbers for Republicans until recently, when national polls have shown a more positive trend for the GOP.

Back in December, Democrats were confident in their ability to take over the House with what was then almost a double-digit generic ballot advantage, at 9.75 points, according to four key national polls: Marist, Reuters/Ipsos, Quinnipiac and Economist/YouGov. Definitely wave territory. Some polls had the generic in the mid-teens for Democrats, but they were likely outliers.

Now, fast-forward to these same four polls, taken March 16-21. Three-and-a-half months later, the Democrats’ generic ballot lead has been cut almost in half, coming in at around 5 points. That’s still a reasonable number for them, but the trend line isn’t in the right direction for a blue wave.

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A little context

When it comes to assessing the generic ballot, some context is in order. In the nine congressional elections from 2000 forward, the actual election results ranged from the Republicans’ best performance in 2010, when the GOP won the national congressional vote by 6.8 percent, to the Democrats’ best performance in 2008, when they won with 10.8 percent.

If you narrow the national congressional vote down to recent elections that changed the House majority, the critical threshold for whether Congress will flip seems to be 6 percent.

In 1994, the Republican margin was 6.8 percent. In 2006, the Democratic margin was 8 percent, and in 2010, the Republicans took control with another 6.8 percent.

That doesn’t mean Democrats couldn’t win the House with a 4- or 5-point lead in the generic ballot, but actual past election results suggest that 6 points is the more likely threshold for Republicans to be in danger of losing the House.

When using the generic ballot to make assumptions, it’s important to remember that assumptions also underlie the generic ballot, which explains the differences often seen in contemporaneous surveys.

Several factors can affect the generic ballot results. Clearly, individual polls have different methodologies.

Start with a poll’s party identification (ID), determining how many Republicans, independents and Democrats should be represented in the sample. We know that about 90 percent of Democrats vote for the Democratic congressional candidate. Same goes for Republicans. So the party makeup in the poll has a huge impact on the generic. If one party or the other is oversampled, that party will have an advantage in the generic.

When we see a major shift in a national survey, the first question we ask is whether the party ID makeup has changed from the previous survey, especially if that change is similar to a change in the generic. One of the challenges for the political community is the failure of some major polls to provide a clear picture of actual party ID, which makes it difficult to assess the credibility of the survey’s results, including the generic.

In other polls, oversampling a party, one way or the other, can be obvious but is released without explanation. This, too, has become one of the most contentious arguments in the polling world because estimates for party ID can be based on the sample itself or on other sources that range from exit poll data to voter rolls.

Maybe the best advice in assessing the accuracy of party ID is “buyer beware.”

A second factor that affects the generic ballot is a poll’s likely voter screen, which represents, in essence, the pollster’s best guesstimate on who is likely to turn out to vote in November. This is a tough element to call. The polling entity may be using their best understanding of what the turnout could look like, but they are introducing this bias into the survey methodology.

This bias isn’t meant negatively or positively. It’s just a fact. In thinking through voter turnout scenarios, pollsters need to consider a wide range of possibilities.

For example, should we assume 18-year-olds are going to vote in higher numbers this year because of the gun control debate? Should we assume turnout is going to be higher overall because of the Resistance movement or because of recent special election results? Are women more or less likely to turn out this election? Are the people who voted for Trump, working-class whites, still enthusiastic enough to vote off-year? If the economy significantly improves, will we see a better turnout among Republicans or center-right independents?

‘A fluid environment’

The answers to these and other questions will determine voter turnout, which in turn creates a fluid environment. This is a significant challenge when implementing a likely voter screen and will influence the poll’s results, including the generic. A word of warning: Because of this dynamic, voter screens can be off the mark. Using likely voter screens in congressional district races in 1998, Republicans thought they would pick up between 20 and 30 seats. They lost five. The actual ballot nationally favored Republicans by 1.2 percent.

Between now and November, there will be much speculation about the generic ballot and what it “predicts” for control of Congress. Political context and survey methodology should always be taken into consideration when putting eggs in the “generic ballot” basket.

The generic is a starting point when assessing the national political environment at a particular moment. From there, as campaigns and their pollsters look at the district level, other factors come into play — the candidate and the specific dynamics of the district. But as political canaries go, with a little caution, the generic ballot test can give us an idea where things could be headed.