Fair political polling is a statistical balancing act
Polls darken for Democrats
Trump has pulled into the lead in Florida and Ohio, two crucial states where he has trailed Clinton for most of the race, and several states that once looked out of reach for Trump - Colorado and Virginia, among them - suddenly appear competitive.
One survey showed Trump swinging to a lead in Nevada, a state that President Obama carried with ease during both of his presidential campaigns. And a poll of Iowa, which has only gone for the GOP nominee once in the last seven elections, found Trump ahead by 8 points.
The swing in national polls is equally dramatic.
While Clinton led Trump by an average of 7.6 percentage points one month ago, her advantage is now down to a meager 1.8 percent, according to the RealClearPolitics average.
"No question there's a movement toward Trump right now," said Monmouth University pollster Patrick Murray. "When the media is focused on one candidate over the other, it's generally negative. The media has been focused on Clinton and her health, and Trump smartly did not try to steal the limelight from her."
The shift in the polls comes amid a brutal stretch for Clinton, who started last weekend by lumping half of Trump's supporters into a "basket of deplorables" and then suffered a dramatic health scare while leaving a 9/11 memorial in New York City, only to later reveal a pneumonia diagnosis.
While the Clinton campaign has showed no public signs of panic, it is bringing the party's heavy artillery to Ohio, dispatching Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.) to campaign in the state.
Democrats can take comfort in the Electoral College map, which gives Trump a narrow path to the necessary 270 votes. To win, he will likely have to pick off a blue state like New Hampshire or Pennsylvania, where he is still behind.
Yet the race is unquestionably moving into toss-up territory as Trump and Clinton prepare for a momentous debate on Sept. 26.
"The first debate is pivotal for Clinton if she's going to arrest this drip and recover," said Geoffrey Skelley, a polling analyst for University of Virginia's Center for Politics. "If she doesn't, then this race stays on a knife's edge. It's gone from being unlikely that Trump could win, to a slightly uphill climb for him."
A CBS News-New York Times poll released Thursday found Trump and Clinton locked at 42 percent support nationally.
Only 43 percent of Clinton's supporters say they're excited about casting a ballot for her, compared to 50 percent of Trump's supporters who are excited to vote for him. More than a third of young voters - a diverse group that leans left and formed a key part of the Obama coalition - are supporting a third-party alternative over Clinton.
Election handicappers are taking a wait-and-see approach before declaring a fundamental shift in the race, believing it's still likelier that Clinton will win enough battleground states to take the White House.
Forecasting models from the University of Virginia's Center for Politics and Frontloading HQ still show Clinton with a significant Electoral College advantage.
U.Va.'s model has Clinton running the table on the battleground states to win 348 electoral votes, which would be slightly better than Obama's showing against Republican Mitt Romney in 2012.
But FiveThirtyEight's forecasting model, run by data guru Nate Silver, finds Clinton's likelihood of victory has plummeted from nearly 90 percent in August to 61 percent.
In the FiveThirtyEight forecast, Clinton ekes out a popular vote victory by 2.3 points and earns 290 electoral votes, compared to 246 for Trump. That's a deficit that can be closed by moving two battleground states from Clinton's column into Trump's.
"I still think Clinton has a slight advantage," said GOP pollster David Winston. "But it has definitely tightened up; this race is extremely volatile."
Republicans have seen this movie before and are skeptical.
This week, conservative commentator Noah Rothman tweeted out a bevy of headlines from October of 2012 declaring that Romney had seized momentum and opened up a lead over Obama in the battleground states.
On Election Day, Obama out-performed the polls on the strength of his superior get-out-the-vote effort. He coasted to reelection, winning every battleground state except for one.
Democrats will once again have the advantage in money and ground game in 2016.
But pollsters caution that there is no analogue for the 2016 race.
Analysts have never had to handicap a race with two candidates as historically unpopular as Trump and Clinton. There are still an unusual number of undecided voters, and interest in the third-party candidates remains a wild card.
Those variables have pollsters struggling to forecast turnout, and importantly, the likely make-up of the electorate.
The uncertainty has fueled debate over polling methodologies as outlets have turned from sampling registered voters to screening respondents for those they believe are likeliest to vote.
"The incredible negatives these candidates have makes it very difficult to determine what a likely voter looks like," said Winston, the GOP pollster.
Earlier in the cycle, Clinton outperformed Trump among likely voters. Now, the pendulum has swung in favor of Trump.
In the Monmouth University survey of Nevada that showed Trump ahead by 2 points, pollster Murray said he screened out a significant number of Hispanics - who presumably would have supported Clinton - because they answered questions in a way that indicated they were less likely to vote.
A Bloomberg poll of Ohio that found Trump ahead by 5 points among likely voters put the electorate at 43 percent Republican and only 36 percent Democratic.
"Our party breakdown differs from other polls, but resembles what happened in Ohio in 2004," pollster Ann Selzer, one of the nation's best pollsters, told Bloomberg. "It is very difficult to say today who will and who will not show up to vote on Election Day. Our poll suggests more Republicans than Democrats would do that in an Ohio election held today."
Pollsters don't know whether minority voters and college-educated whites will turn out for Clinton because they fear a Trump presidency. They doubt the third-party candidates will pull their current level of support, but are unsure if those voters will stay home or move to a major party candidate. And they don't know if Trump's enthusiasm advantage is enough to overcome the Democratic turnout machine.
"It's hard to know what the polls mean right now because the vast majority are motivated to vote against the other candidate," Murray said. "We've never had a situation like this. It's unprecedented. You can't compare it to anything in modern times."