"Now recent polls have shown that I'm beating Hillary Clinton," Trump said during his speech Tuesday night, "and with all of her many problems and the tremendous mistakes that she's made -- and she has tremendous mistakes -- we expect our lead to continue to grow and grow substantially."

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But that's not happening. Even before the Democratic Party has united around its nominee, Hillary Clinton, Trump's improvement in polling against Clinton appears to have stalled -- because support for him has dropped.

You can see it in the Real Clear Politics average of recent polls. Trump gained against Clinton, but Clinton started to pull away again at the end of last month. And then, that dip for Trump.

Most of that is because of a Fox News poll released Thursday, which shows that Trump went from beating Clinton by 3 points last month to trailing her by 3 this month. That's all within the margin of error, but what's interesting is the change: Clinton hasn't moved since May. Trump dropped 6 points.

If we look at how demographic groups have shifted over time, we have to first acknowledge that there are some constants. Black voters and Democrats love Clinton; Republicans love Trump. There are caveats to that: support for Clinton among Democrats has slipped in each of the past two polls, and now Republicans like Trump by about the same margin as Democrats like Clinton. But those groups are so much more partisan that it makes everyone else hard to see.

So let's take them out. Here is that middle cluster, organized by demographic pairing.

Two things worth paying attention to there: young people and independent voters.

Let's start with young voters. Here's that frame of the animation.

Notice what's happening there? Young people have moved more and more to the middle in Fox News polling since April, even as other groups move up and down with the overall trend. And who do young people support in the Democratic contest? Bernie Sanders, by a wide margin.

We've seen this trend in Post/ABC News polling, too. Look at the favorability ratings for Clinton among voters under 30. Used to be about half-and-half. In the two most recent surveys, though, Clinton is viewed much more unfavorably than Trump.

One of the things we'd expect to see is that those numbers will fall as the party reunifies (potentially with President Obama's help). If younger voters do swing back to Clinton, that will move Clinton's numbers up -- even as Trump's may continue to go down.

Speaking of Sanders voters: The other group that backed him heavily was independents. That's a group that helps explain a lot of why Trump's numbers plunged over the past month. (Worth noting that the Fox News survey was conducted earlier this week, as Trump's comments about the judge in the Trump University case -- which even Republicans labeled "racist" -- were in the news.) Last month, Trump had a wide lead with independents. This month, it's much smaller. In a three-way race, including the Libertarian candidate, Gary Johnson, Johnson gains as Trump fades.

Johnson and Clinton are basically tied among independents, in fact. How many of those independents are the same independents that powered Sanders's insurgency is hard to say. But we might expect, again, that Clinton would see some gain from that group as the Democratic contest wraps up. (It's also hard to say how real the Johnson support is. He may be benefiting heavily from being not named Trump or Clinton, and third-party candidates tend to do better in polls than on Election Day.)

That's speculative. What we can say without qualification is that Trump's apparent improvement in the polls against Clinton has stalled, thanks to some wavering in his support. Will this prompt a moment of reflection for Trump, inspiring him to point out in his speeches that he's hit a rough patch and that the polls aren't showing him solidly in the lead (not that they ever did)?