Harris Ahead in Pennsylvania and Tied Nationally? Unpacking an Unexpected Result.

We have our first two polls since last week’s presidential debate: one national poll and one poll of Pennsylvania.

Combined, they’re a bit of a puzzle.

In the national poll, Kamala Harris and Donald J. Trump are tied among likely voters, 47 percent to 47 percent — a slight gain for Ms. Harris since our last national survey, taken immediately before the debate.

At the same time, Ms. Harris had a four-point lead in a New York Times/Philadelphia Inquirer/Siena College poll of Pennsylvania, 50 percent to 46 percent.

Before getting into the head-scratching details, let’s start with the big picture:

• Not much change since the debate. Despite a strong debate performance, Vice President Harris did not gain much ground compared with our last polls of the nation and Pennsylvania. The poll is full of evidence that our respondents thought she did well in the debate — and that Mr. Trump did poorly — but it hasn’t made a big difference, at least for now and at least in our polling.

• Pennsylvania, Pennsylvania, Pennsylvania. Ms. Harris may not have gained much, but her campaign will surely be happy with the result in Pennsylvania. The national result, on the other hand, is quite favorable for Mr. Trump (that’s the head-scratching part we’re about to examine). But our elections are decided by the Electoral College, and no state figures more prominently in the electoral math than Pennsylvania.

Now let’s consider our puzzle: a clear lead for Ms. Harris in Pennsylvania, but a tie nationwide? This is unexpected. Four years ago, President Biden won the national vote by 4.5 percentage points, but won Pennsylvania by just 1.2 points. Similarly, our poll averages have shown Ms. Harris doing better nationwide than in Pennsylvania. This poll is nearly the opposite.

Usually, I’d say that this is probably just statistical noise — the inevitable variation in poll results inherent to random sampling. And it might well be, as we shall see. But I think it’s hard to assume that this is simply noise, for two reasons:

• It’s what we’ve shown before. It’s easy enough to dismiss any single poll result as a statistical fluke. But we’ve now found similar results in our last two polls of the nation and Pennsylvania.

• This is becoming a trend among high-quality pollsters. Yes, our poll average shows Ms. Harris doing better nationally than in Pennsylvania, but if you focus only on higher-quality polls (which we call “select pollsters” in our table), the story is a bit different. Over the last month, a lot of these polls show Ms. Harris doing relatively poorly nationwide, but doing well in the Northern battleground states.

A note on “select” pollsters: To be considered select on our poll averages page, pollsters must meet two of three criteria: a track record of outperforming other pollsters; a transparent methodology; the use of a method that has a chance to reach most or all potential voters. This isn’t a perfect approach (it omits some pretty good polls, and it includes some that aren’t great), but it includes most of the heavy hitters in the industry and it weeds out much of the junk.

When you focus on these higher-quality polls, you get a surprising picture: There are a lot of good polls for Mr. Trump nationally, and a lot of polls showing Ms. Harris doing relatively well in the Northern battleground states like Pennsylvania.