Every day, it seems, another warning light goes off on the GOP’s dashboard, a blinking reminder of their ensuing troubles for 2018. There are a record number of GOP retirements — including that of House Speaker Ryan. Democratic challengers are outraising GOP incumbents. And, of course, there's the most recent under-performance of a GOP candidate in Arizona’s 8th district special election.

But, If a so-called "blue wave" is about to hit in 2018, why isn’t the generic ballot showing a bigger margin for Democrats? The latest Real Clear Politics average shows Democrats with a 6.5 percent lead. The FiveThirtyEight.com average has Democrats with a 6.9 percent lead. If Democrats are cruising to victory in the fall, why does the generic not look more like it did over the summer when it showed Democrats with a double-digit lead?

Here’s my best guess. First, we tend to spend too much time looking at the margin instead of the vote itself. For example, the Quinnipiac poll in March had Democrats up 10 points. In April, that lead was down to just 3 points. The headline: Democrats lose their lead! But, let’s take a closer look at what actually changed between March and April. In March, 48 percent said they’d like to see Democrats win control of Congress to just 38 percent who said they’d want Republicans in control. In April, 46 percent wanted to see Democrats in control (a slight 2 point drop), while 43 percent picked the GOP (a more impressive 5-point improvement).

What does this mean? It means that Republicans are "coming home." Even in a terrible year for the GOP, they are not going to perform much worse in the national vote than 43-44 percent. In 2006, for example, Republicans took 44 percent of the national House vote, even as many polls leading up to Election Day showed Republicans in the high-30’s. In 2008, an even more politically horrific year for the GOP, Republicans garnered 43 percent of the national House vote. In both cases, Republican voters, many reluctantly, "came home" to the GOP in the end. What’s happening now is that these voters are coming home sooner. Given our intense polarization, and a president and a news media that fans those partisan flames, this shouldn’t be all that surprising.

The question for the fall, of course, is where those who currently put themselves in the "undecided" category break. We know these voters are much less engaged in politics. They are less attached to party and partisanship. We lump this group into the category called "independents." And, here’s what we know about them: they don’t like Trump. Overall, about one-third to 40 percent of self-described independent voters approve of the job Trump is doing as president. And, as we know, how you feel about the president is correlated very closely to how you vote in a mid-term election.

In the latest Marist/NPR/PBS poll (April 10-13), for example, Trump’s job approval rating among independents is 38 percent. On the generic ballot question in that same poll, the congressional Republican gets 32 percent of the independent vote. A late April Quinnipiac poll showed Trump with a 33 percent job approval among independents, and 36 percent of independents say they will vote for a Republican in the fall.

So, let’s move this forward. We can make a pretty good case that undecided independent voters are ultimately going break along Trump approval ratings. So, if Trump’s job approval rating among independents is at 40 percent going into the election, it’s fair to assume that 40 percent of the undecided independent vote will go to the Republican candidate. It could be a bit better or a bit worse. In the Marist poll, for example, 32 percent of independent voters are undecided on their vote for Congress. If those 32 percent ultimately broke along Trump approval ratings, it would mean that a Republican candidate could expect to get 44 percent of independents.

However, a 44 percent showing among independents is a bit better than where Republicans were in 2006 and 2008. According to the exit polls, Republicans took just 39 percent of the vote among independents in 2006. In 2008, Republicans took 43 percent of the independent vote (a 4 point improvement from their 2006 showing), but Democrats also made up a larger share of the electorate in 2008 than in 2006. In 2006, the share of the electorate that was Democrat was 38 percent to 36 percent Republican. In 2008, that gap grew to 7 points (40 percent Democrat to 33 percent Republican). In other words, the closer to even that Democrats and Republicans are as a percentage of the electorate, the better Democrats need to do among independent voters.

Bottom Line: Pay more attention to Trump job approval rating among independent voters than the overall margin in the congressional ballot. Unless or until Trump improves his standing among independents, Republicans in Congress shouldn’t expect to see an improvement in their ballot standing with them.

It's also important to remember that the generic ballot question is just that: generic. There are some districts and states where Trump’s job approval rating among independent voters is going to be better and some where it is going to be worse. Intensity of support/opposition to Trump among independents matters as well. The latest Marist poll finds 40 percent of independent voters 'strongly' disapprove of the job Trump is doing, compared with just 20 percent who 'strongly' approve. This means that the overall independent pool of voters could be more anti-Trump than the top line numbers suggest.

We also know that assumptions about turn-out matter too. For example, if Republicans have a bigger vote share in a state or district, they may be able to withstand a poor showing among independents. This is why Republicans are concerned about GOP apathy and Democratic intensity. But, in a swing district, even increasing GOP voter participation may not be enough if independents break decisively for the Democrats.