Fair political polling is a statistical balancing act
Polls tighten in presidential race
Clinton opened her largest margin on Aug. 9, when she had a 7.6 percentage point advantage over Trump, the Republican nominee, in the RealClearPolitics average of national polls. At the time, she was consistently reaching 50 percent support.
But Clinton's lead has shrunk since then, to 4.3 points in the RealClearPolitics average, and she's fallen short of the 50 percent mark in the last six national polls. She has settled into the mid-40 percent range, presenting an opportunity for her Republican rival.
But despite the improving picture for Trump, Clinton remains the favorite to win.
She holds a 4.3-point advantage nationally over Trump in the RealClearPolitics average of polls and leads almost across the board in the battleground states that will decide the election.
Trump's path to the necessary 270 electoral votes is exceedingly narrow, with a handful of swing states - Colorado and Virginia among them - already appearing out of reach.
And while the GOP nominee is running competitively in Florida and Ohio, Trump must also win Pennsylvania, which looks like a steep climb.
For Trump, the deficit in polling has at least reached a level he can overcome as he enters the post-Labor Day sprint.
Clinton is being dragged down by awful favorability ratings, which have prevented her from running away with the race.
"It's not in the bag for her yet," Patrick Murray, the polling director at Monmouth University, told The Hill.
An NBC News-SurveyMonkey poll released Tuesday found Clinton's lead falling from 6 points earlier this month to 4 points. In that survey, Trump reduced Clinton's lead among independents by half, from 8 points to 4 points.
Trump has also closed the gap in Reuters-Ipsos and Monmouth polls released this week, although he still trails by 3 points and 7 points, respectively.
Pollsters interviewed by The Hill say that Clinton's polling bounce after her party's national convention was aided and perhaps magnified by Trump's feud with the family of a slain U.S. solider.
They see a race returning to an equilibrium in which Clinton holds a statistically significant advantage, but opportunities exist for Trump to win over Republican holdouts and independents.
"They're both hitting ceilings of support because of their hugely negative favorability ratings," said GOP pollster David Winston. "The challenge is figuring out how to get people who don't like them at all to get out and vote for them."
Electoral experts agree that the map looks favorable for Clinton.
Forecasting models from the University of Virginia's Center for Politics and Frontloading HQ have Clinton winning 347 electoral votes, which would be slightly better than President Obama's showing against Republican Mitt Romney in 2012.
Both models have Clinton running the table on the battleground states Obama won in 2012, plus winning in North Carolina, where Romney narrowly prevailed.
Several swing states appear to have gotten away from Trump.
Polls show Clinton leading by double digits in Colorado, Wisconsin and Virginia. Clinton is ahead by 9 points in the only poll of New Hampshire released this month.
Furthermore, Trump only leads by 1 or 2 points in Missouri and Arizona, states that traditionally have been safely in the Republican column. Trump and Clinton are locked in a virtual tie in Georgia, which has gone red in presidential races for the last 20 years.
Things look better for Trump in Nevada, Iowa and North Carolina. He trails in all three, but is within the polls' margins of error.
The election is likely to hinge on Florida, Ohio and Pennsylvania - and pollsters say Trump must win all three.
In Florida, Clinton leads by 2.7 points in the RealClearPolitics average, down from 4.5 points in early August.
In Ohio, Clinton's lead in the RealClearPolitics average is 3.8 points, down from a 5-point advantage in recent weeks. An Emerson University survey released on Monday found the nominees tied at 43 percent.
Pennsylvania will be the toughest state of the group for Trump. Clinton leads there by 8 points in the RealClearPolitics average.
An Emerson survey put Clinton's lead at only 3 points there on Monday, but pollsters will need more data like it before they see the state getting more competitive.
A Monmouth poll released on Tuesday found Clinton ahead by 8 points in the Keystone State, which is more in line with most other recent surveys.
Clinton also holds an 8-point lead in Michigan, another Rust Belt state that Trump has circled as potentially fertile ground for his populist pitch. An Emerson survey released on Monday found Clinton ahead by only 5 points there, while a Suffolk University poll released this week put her lead at 7 points.
"Clinton's battleground advantage is formidable," said UVA Center for Politics analyst Geoffrey Skelley. "Trump basically needs to run the table on the swing states that are favorable for him right now, plus win in Ohio, Florida and Pennsylvania. He'll need his fortunes to improve for that to happen. It's a tough route."
One task for Trump is shoring up votes in his own party.
Clinton routinely receives support from more than 80 percent of Democrats polled, but the latest Reuters/Ipsos survey found Trump with only 73 percent support from likely Republican voters.
Romney got 93 percent GOP support in 2012 and still lost by almost 4 points nationally.
Pollsters say the deficit among Republican voters is largely due to white, college-educated women, a group that traditionally leans conservative but has rejected Trump so far.
Trump also needs to improve his standing among independent voters, which Romney won by 5 points in 2012.
The Trump campaign has argued that his support is being underestimated in polls because those being interviewed have been reluctant to admit they plan on casting a ballot for the controversial candidate.
Pollsters interviewed by The Hill aren't ready to make that leap yet.
"It's an interesting theory, but there's no data to support it," said Winston.
Some pollsters actually believe Clinton's level of support is being underestimated.
Third-party candidates - Libertarian Party nominee Gary Johnson and Green Party nominee Jill Stein - have been pulling more support away from Clinton so far. But few believe either third-party candidate will pull as much support as they're currently getting in the polls on Election Day, potentially adding to Clinton's total.
In 2012, Obama outperformed his standing in the polls by virtue of his superior get-out-the-vote operation.
Clinton will have the money and ground game advantage over Trump in 2016, potentially setting her up to match Obama's showing.
"The thing that could throw all this off is that we're looking at an ahistorical election," Murray said. "We've never seen two candidates who are this unpopular. That's the wild card."