Senate Democrats are polling well. That could help Biden.

As Democrats wring their hands over their poor polling numbers in the presidential race, there is one spot of good news for them: U.S. Senate races. Democratic candidates have led in most recent polls of key Senate races like Arizona, Michigan, Nevada, Pennsylvania and Wisconsin* even as President Joe Biden trails former President Donald Trump in the very same polls.

I went back and looked at every state-level poll that has asked about both the presidential race and a U.S. Senate race over the past six weeks. The table below shows a simple average of these polls by state; as you can see, Senate Democrats are outrunning Biden's margin by an average of 5 percentage points:

It isn't hard to figure out why this is the case. Stop me if you've heard this before, but Biden is unpopular; meanwhile, most Democratic candidates for Senate are popular incumbents, like Pennsylvania Sen. Bob Casey and Virginia Sen. Tim Kaine, and/or have a track record of outperforming the top of the ticket, like Michigan Rep. Elissa Slotkin and Minnesota Sen. Amy Klobuchar. Lo and behold, the only three states where Republican Senate candidates are polling better than Trump are the two states where they are incumbents (Florida and Texas) and Maryland, where the GOP's candidate is popular former Gov. Larry Hogan, who had an astounding 81 percent approval rating among Democrats at the time he left office.

So it's not exactly a mystery why there's a presidential-downballot gap. The more interesting question is what, if anything, that gap means for November. There are basically three theories to answer that:

1. Biden's support right now is artificially low; there are plenty of traditionally Democratic voters who are telling pollsters they support Senate Democrats but, for whatever reason, aren't ready to get behind Biden yet. So Biden's support will rise as we get closer to the election and those voters get on board, leaving both Biden and Senate Democrats in good shape.

2. Senate Republicans' support right now is artificially low. Since most of them aren't incumbents, they aren't as well known as their opponents, but their support will rise as Trump voters learn that they are on the "right team" in these Senate races. Therefore, both Trump and Senate Republicans will do well in November.

3. Senate Democrats are just more popular than Biden, and this gap will persist through the election: Many states will vote for both Trump and a Democratic Senate candidate.

Right off the bat, I'm skeptical of No. 3. If the current polls end up being exactly correct, Democrats would win at least five Senate races in states Trump carries. That wouldn't have been unusual 20 years ago — in 2004, seven states voted for different parties for president and for Senate. But today, split-ticket voting is quite rare. In 2016, every state voted for the same party for Senate that it did for president, and in 2020, every state but one (Maine) did.

The median difference between the presidential and Senate margins in a state has also shrunk drastically over that time — from 20 points in 2004 to just 3 points in 2020.

By comparison, the median difference between the current presidential and Senate polling margins in the 10 states in the table above is 10 points. It's just really hard to imagine that happening in such a polarized political environment.

So if we assume that the presidential and Senate results are eventually going to come into closer alignment, we're left with the question of which party that will benefit. And to answer that, we need to look beyond just the Democratic-Republican margins in recent polls to the actual percentage of the vote that Biden and Trump are getting compared with Senate Democrats and Senate Republicans.

You see, something that margins obscure is the fact that some voters are still undecided at this early juncture — and looking at actual percentages can give you a clue about who those voters might eventually be inclined to support.

Imagine State X, where Trump leads Biden 43 percent to 39 percent — a margin of 4 points — but the Democratic Senate candidate leads the Republican Senate candidate 47 percent to 41 percent — a margin of 6 points. That’s a 10-point difference in the margin, but it’s not because 10 percent of the voters in this state are splitting their tickets between Trump and the Democratic Senate candidate. Instead, Trump and the Republican Senate candidate have roughly the same level of support: around 42 percent. But the Democratic Senate candidate is running almost 9 points ahead of Biden, 47 percent to 39 percent, indicating that a lot of people voting Democratic for Senate haven’t decided who they’re voting for at the top of the ticket yet.

It's plausible that these people could vote for either Biden or Trump in the end; undecided voters, by definition, could go either way. But it is easier to imagine them voting for Biden because they have already expressed willingness to vote for a Democrat — and, again, split-ticket voting has become as rare as platinum. So this scenario would give Democrats reason for optimism.

Well, luckily for Democrats, this is a real scenario: State X is Arizona. And it's a similar story in Pennsylvania and Wisconsin so far — almost all of the presidential-Senate discrepancy in the polls there is accounted for by Biden's soft support among Democratic Senate voters. There's no guarantee that he'll be able to do it, but if Biden can just win over every supporter of Casey, Arizona Rep. Ruben Gallego and Wisconsin Sen. Tammy Baldwin,** he has a good chance of erasing his deficit in those states.

On the other hand, the news is worse for Biden in other states. Biden has lots of room to grow in Nevada as well, where he is polling 9 points behind Sen. Jacky Rosen. But likely Republican nominee Sam Brown is also polling 6 points behind Trump, and if both of them catch up to the leader, the Silver State would be roughly tied, 45 percent to 44 percent.

Meanwhile, in Michigan, it's actually Biden and Slotkin who are polling at the same level (42 percent). Likely Republican nominee former Rep. Mike Rogers, though, is underperforming Trump by 3 points, implying that he could pull ahead of Slotkin in Senate polling once Republicans start learning who he is.

This is, of course, an oversimplified exercise; again, these undecided voters aren't guaranteed to break uniformly for the party that they've expressed support for elsewhere on the ticket. In fact, they're almost certain to not break uniformly — that is, some will vote for the same party for president and Senate, but others will split their ticket.

And, crucially, there are still lots of voters who are undecided about their votes for both president and Senate. Look back at the table above; only two numbers in it are above 50 percent (the 51 percent of Floridians who support Republican Sen. Rick Scott and the 55 percent of Marylanders who support Biden). So even if every not-Biden Democrat and every not-Republican-senator Trump supporter comes home in the end, the election in most states is going to come down to which way a more unpredictable group of voters breaks — the truly undecided.

Geoffrey Skelley contributed research.

Footnotes

*Of course, Democrats can win all these states and still lose the Senate if Republicans flip West Virginia and just one of the following three: Montana, Ohio or the vice presidency.

**Importantly, this is not the same as the supposed "reverse coattails" phenomenon, the idea that the energy and campaign efforts for downballot candidates benefit their party's top-of-the-ticket candidates by driving voters to the polls who otherwise wouldn't have voted. There is little hard evidence that reverse coattails are a real thing.