Just before the first presidential debate Monday night, a new round of polls has been released that should strike fear into the hearts of Democrats.

First off, the national race is looking very tight indeed. Democrats had hoped that Clinton had rebounded after a couple of respected polls last week showed her taking a 6-point lead over Trump. But the newest polls from major outlets — ABC News/Washington Post, Quinnipiac, and Bloomberg — show Clinton up 2, up 1, and down 2 to Trump in a four-way matchup that includes Gary Johnson and Jill Stein.

But what’s probably scarier for Clinton is that occasional polls are now coming out showing Trump leading Colorado — a state that has been a key part of Clinton’s most likely path to victory.

Indeed, this new round of polling has been enough to tip FiveThirtyEight’s “now cast” model, meant to show who would win the election if it were held today, into projecting a Trump win.

Keep in mind that FiveThirtyEight’s other two versions of its model still show an extremely narrow Clinton edge, and every other major model still gives Clinton the edge too, as you can see at the Upshot. So overall, Trump’s not the favorite yet.

But the Colorado news especially helps suggest a plausible path to an Electoral College victory for Trump, which he hasn’t had in some time, and suggests that what some have dubbed a “firewall” for Clinton may no longer be so secure.

Why Colorado could tip the election to Trump

Colorado had long been considered a solidly Republican state in presidential elections, but a growing Latino population helped Barack Obama win it twice, and Trump’s apparent weakness among college-educated white voters seemed to give Clinton a solid lead in the state earlier this year.

Indeed, Clinton’s team thought she had basically locked up Colorado, and she stopped her TV advertising in the state this summer.

But though there have only been a few Colorado polls this month, the evidence is piling up that the race there has dramatically tightened.

Clinton has led three of the six polls in Colorado this month, but she’s only had a solid lead in one of them. In the four-way matchup, Franklin and Marshall puts her ahead by 7, Quinnipiac by 2, and CBS/YouGov by 1.

Meanwhile, the other three polls have shown Trump taking the lead there. Polls from Emerson and Gravis/Breitbart this month have shown Trump leading by 4. Many commentators dismissed them, since Emerson doesn’t call cellphones, Gravis isn’t very highly regarded, and Breitbart ... well, Breitbart may not exactly be the most unbiased source. But a new CNN/ORC poll just released Monday, showing Trump leading by 1 in the four-way matchup, is tougher to dismiss. Clinton’s still ahead a bit on average, but Colorado is clearly not looking as certain for her as it did in August.

For the past two months, Clinton’s easiest path to victory has appeared to be through winning six swing states — Michigan, Wisconsin, Virginia, Pennsylvania, New Hampshire, and Colorado. It’s not her only path to victory, but her apparently durable leads in those six states helped her supporters feel confident even as the national race grew closer.

But if Trump does pull ahead in Colorado, well ... that could be the ballgame. This is a pretty plausible map to a Donald Trump victory based on current polling, with Colorado’s 9 electoral votes putting Trump over the top.

Now, Clinton could also win by pulling either Florida or North Carolina, both of which look to be extremely close, away from Trump here.

But there are other potential problems in Clinton’s supposed six-state firewall. In Pennsylvania, Clinton’s lead also seems to have shrunk in recent polls from Muhlenberg/Morning Call and CNN/ORC. And even in Wisconsin, the most recent high-quality poll — from Marquette Law — shows Clinton up by just 3 over Trump in the four-way matchup. (Clinton’s lead in Virginia, another once-swing state where she stopped advertising last month due to confidence in a victory there, does appear to be holding up, though.)

So to get a sense of whether the Clinton team is spooked by this news from Colorado, keep an eye on whether they start up ads in the state again or whether Clinton herself schedules a trip there.

Watch: Democrats are in trouble even if Clinton wins