(CNN) Hillary Clinton and Donald Trump each campaigned Thursday in North Carolina, which is emerging as perhaps the state on which the 2016 presidential race turns.

Clinton has held a small but persistent lead in the state in recent weeks. A Quinnipiac University poll out Wednesday showed Clinton ahead, 48% to Trump's 46%.

It's made North Carolina's 15 electoral votes Democrats' best chances of winning a state Mitt Romney carried -- albeit by just 2 percentage points -- in 2012.

And for Trump, whose path to 270 electoral votes is already precarious, North Carolina is essentially a must-win -- a test of whether he can turn out working-class white voters and survive organizational and TV ad spending disadvantages to the Clinton campaign.

If Trump can't do it in North Carolina, he has little shot of remaining within striking distance of Clinton and pulling off the same feat in some combination of states like Colorado, Nevada, New Hampshire, Pennsylvania and Wisconsin -- all even more difficult to win.

Here's why North Carolina matters so much -- and what to watch there in the days ahead:

By the numbers: Why Trump has to have it

Every Trump path to 270 electoral votes starts with one reality: There are four battleground states he simply cannot afford to lose.

"Our path to victory is through Florida, North Carolina, Ohio and Iowa. And we believe 100% that we're winning all four of those. And once we do that, then we put ourselves in a position to win one of those other swing states," Trump deputy campaign manager David Bossie said this week.

North Carolina is the only one of those four that Romney won in 2012 -- yet it's also the one where Clinton's poll numbers are the best in 2016.

Say Clinton wins North Carolina. Trump could win every other Romney state -- plus Florida, Ohio and Iowa -- and that still only puts him at 244 electoral votes.

He's competitive elsewhere -- in states like Nevada, Colorado in New Hampshire. He's bet big on Pennsylvania, and made a late push in Wisconsin.

But none of those states alone would get him to 270 without North Carolina -- and in most cases, even a combination of two of them wouldn't get him there.

There's also the reality that Clinton outperforms Trump in most other battleground states by more than she does in North Carolina. If he loses North Carolina, it likely means he's already lost the election.

African-American turnout

One trend that has alarmed Democrats is a dip in African-American turnout across the country -- particularly in states like North Carolina, Georgia and Florida -- compared to the 2008 and 2012 elections.

A surge in Latino voters has helped make up for it in some states, particularly in the West.

But polls show the demography of the electorate is crucial: The more ethnically diverse a state's voters are, the better it is for Clinton; the whiter the state's electorate, the better for Trump.

Clinton is leaning on President Barack Obama and First Lady Michelle Obama to make the case to their African-American supporters that 2016 is just as important as the last two elections -- particularly in North Carolina, a state that had been reliably Republican before Obama carried it in 2008 and nearly repeated the feat in 2012.

"If the African-American vote is strong in North Carolina, then you will decide this election," Obama said in an interview with North Carolina-based WQMG radio on Thursday, adding that Clinton will win if the state's black turnout matches its 2012 numbers.

"Your vote from '08 and '12 will be negated if you don't vote this time out. This is what seals the deal," Obama said. "Show the country and the world this isn't just about me and my campaign or Michelle, but it's about a community that is empowered and wants its voice heard."

Obama also raised the stakes Wednesday during a campaign stop on Clinton's behalf in North Carolina, saying: "I hate to put a little pressure on you, but the fate of the republic rests on your shoulders. The fate of the world is teetering, and you, North Carolina, are going to have to make sure that we push it in the right direction."

Voter purge lawsuit

The backdrop for North Carolina's 2016 election is a lawsuit that voting rights advocates filed this week alleging that at least three counties have purged their voter rolls of a disproportionate number of African-American voters.

The NAACP filed the lawsuit, arguing that boards of elections in Beaufort, Moore and Cumberland counties have canceled thousands of voter registrations after a small number of individuals challenged the registration of approximately 4,500 voters based "exclusively on mass mailings that were returned as undeliverable."

They argue the "en masse" cancellation was done in violation of the National Voter Registration Act that prohibits systemic voter removal programs within 90 days of a federal election.

On Wednesday, US District Judge Loretta Biggs said North Carolina's process of removing voters sounds "insane" and "like something that was put together in 1901." But she has not yet ruled, with just days until the election.

Early voting

In 2012, 61% of North Carolina voters cast their ballots before Election Day -- a number expected to rise this year.

So far, early voting numbers, based on a breakdown of the party registration of those who have voted already, show Democrats have reason for worry.

African-Americans made up 28% of the state's early voters at this point in 2012 -- but are just 22.7% so far this year, according to data from CNN's partner Catalist

White voters, meanwhile, make up 73% of the state's electorate to date, compared to 67% at this point four years ago.

Overall, though, Democrats still have a lead of 243,000 ballots cast so far in the early vote.

But that's off their 2012 pace -- when Democrats led at this point by 307,000 votes, and still lost the state to Romney.

Surrogates abound

Perhaps the best evidence of North Carolina's importance is how much attention each campaign's top surrogates are paying it.

Ivanka Trump was in the state Wednesday for an event. Clinton's campaign has sent the Democratic nominee herself, husband Bill Clinton and the Obamas there.

It's also where Hillary Clinton held her first joint event with Michelle Obama of the 2016 campaign.

The candidates are putting a particular emphasis on targeting millennials and North Carolina's swelling ranks of college graduates.

President Obama's event Wednesday was at the University of North Carolina in Chapel Hill. The joint Hillary Clinton-Michelle Obama event was held at Wake Forest University in Winston-Salem.