Thousands had gathered in an outdoor field on the campus of the University of North Carolina at Chapel Hill, braving the sweltering sun for hours ahead of what was likely Barack Obama’s final visit to the area as president.

With less than a week remaining until election day, Obama’s speech campaigning for Hillary Clinton was marked by a palpable sense of urgency amid a tightening race. But as the US president delivered what has become his routine stump speech for Clinton on Wednesday – branding Donald Trump a threat to democracy and attacking the judgment of Republican lawmakers who support their nominee – he invoked a lesser known name before the fervent crowd.

“I want you think about a woman named named Grace Bell Hardison,” Obama said.

Hardison was a 100-year-old black woman who had lived in the same town in North Carolina her entire life. Last month, her voter registration was challenged in her home of Beaufort County in what officials called an effort to “clear up” the voting rolls. Of the list of 138 purged voters that included Hardison, 92 were black and registered Democrats.

“That didn’t happen by accident,” Obama said. “There was a time when systematically denying black folks the right to vote was considered normal as well.”

Trump, he noted, was calling on supporters to monitor “certain areas” at the polls.

“They’re just out in public saying we’re going to try and suppress the African American vote on election day, or the youth vote on election day.”

This was a seeming reference to a Bloomberg story from last week in which an unnamed Trump official was quoted as saying: “We have three major voter suppression operations under way,” aimed at idealistic white liberals, young women and African Americans.

The critical battleground of North Carolina is in many ways the ground zero of concerns over access to the ballot among black voters.

Republicans in the state have purged thousands of voters from the rolls, the majority of whom are black Democrats. The NAACP filed a lawsuit this week, stating that the practice of purging voter rolls up to 95 days before an election was a violation of federal law. The US justice department also issued a sharp warning to the election boards in the relevant counties of Beaufort, Moore and Cumberland.

Penda Hair, an attorney representing the the North Carolina NAACP, said the counties in question were allowing “individual private vigilante residents” to challenge voters based on mailings that were unregulated by the state. The private citizens were sending mailings that would come back as undeliverable, which they would then use to challenge the residence of the voters they wanted the county to purge from the rolls.

To those in North Carolina, the pattern of purging voter rolls is reminiscent of efforts to curtail the black vote through strict voter ID laws struck down by a federal court earlier this year for targeting African Americans “with almost surgical precision”. The supreme court denied a request in August to reinstate the law.

For Democrats, the issue threatens to dampen turnout among one of its core constituencies. African Americans were instrumental in twice propelling Obama to the White House and have been aggressively courted by Clinton throughout her campaign over the last 18 months.

With the clock ticking toward 8 November and early voting already under way, Clinton and her top surrogates have devoted ample time to shoring up support among black voters.

Obama won the state of North Carolina in 2008 by just about 14,000 votes, and went on to lose there to Romney in 2012 by roughly 92,000 votes. As Obama and his wife Michelle have implored voters while campaigning for Clinton across the country: just two votes per precinct hold the potential to sway the election away from Democrats.

The Obamas, who have emerged as Clinton’s most powerful surrogates, have tailored their message in particular to appeal to both black and young voters. Seeking to counter a potential enthusiasm gap toward Clinton among such key demographics, the president framed efforts by Republicans to suppress the vote as precisely what should motivate them to exercise their political power.

Early voter turnout worries Democrats who fear 'enthusiasm gap' for Clinton Read more

“How are we going to betray all those who worked so hard, risked everything for the vote so that we could pull the lever and we’re not going to vote? What’s our excuse?” Obama asked, after revisiting the civil rights struggles of the past that included registered black voters being beaten Mississippi.

“It was not that long ago that folks had to guess the number of jellybeans in a jar, or bubbles on a bar of soap, or recite the constitution in Chinese in order to vote.”

An Associated Press analysis of early voting data in North Carolina found that black voters have cast 111,000 fewer ballots than at the same time in 2012. Democrats cited voter suppression tactics by Republicans as part of the challenge, noting an initial reduction by the GOP-led election board in the number of early voting sites.

More voting sites have since opened, prompting turnout to “skyrocket”, according to a memo distributed by the Clinton campaign on Wednesday.

The field report noted that 15,000 African Americans had voted in Guilford County over the last three days, where new early voting sites opened late last week, marking a 60% higher turnout among black voters than during the same three-day period in 2012. One of every three ballots cast in the county were by black voters, Clinton’s campaign noted.

Both Trump and Clinton returned to North Carolina on Thursday, while Obama took his pitch to Jacksonville, Florida, where early-voting returns pointing to an apparent loss of enthusiasm among black voters are fueling similar Democratic fears within that Trump could yet snatch victory in another crucial swing state.

Exit polls from 2008 and 2012 showed 95% of Florida’s black voters backing Obama. But they have not offered the same level of support for Clinton in the numbers expected, according to an analysis of early in-person voting data, angering a prominent African American Democratic congressman who said he had been “screaming for months” for the Clinton campaign to spend more money on the black community in the state.

“They’re not doing enough in the black community. I have been screaming for months about this and nothing changed and now look what’s happening,” said congressman Alcee Hastings, whose south Florida district incorporates almost all of the majority black precincts of Fort Lauderdale and West Palm Beach, told Politico.

Clinton’s staff has escalated its presence in the Sunshine State in the campaign’s final days, with several appearances by the Democratic nominee as well as prominent surrogates in areas with significant black populations. They include Obama’s appearance on Thursday in Duval County, where one in three registered voters is African American, and two addresses in three days by Clinton to overwhelmingly black crowds at a Baptist church and a civil rights memorial park in Fort Lauderdale, focused on themes such as reform of the criminal justice system and gun control.

“The black vote is very important for Clinton’s chances; the higher the turnout the better her chances are,” said Matthew Pietryka, associate professor of political science at Florida State University.

“They need to be rallying the troops and African Americans are historically and consistently strong supporters of Democrats,” he said. “Mobilization through churches is particularly effective, so being short on time and with early voting underway, it’s an effective way to get out the vote.”

The worry for Clinton, who has seen her four-point lead over Trump in Real Clear Politics’ Florida average of polls turn into a dead heat in only 10 days, is reflected in a comparison of this year’s early-voting returns so far with those from 2012.

While the official state figures, analysed by elections expert Dan Smith of the University of Florida, do not show which candidate the voter supported, they indicate that only 16% of early in-person votes cast in Florida by Sunday night, after seven days, were by African American voters. In 2012, after only two days of early voting, the figure was 25%.

Although the makeup of Florida’s electorate has changed since 2012, Smith believes the lower than expected turnout – which matches a downturn in the early black vote in at least seven other states – is particularly worrying for Democrats in a state where recent polls suggested African American voters favoured Clinton over Trump by a 93 to 4 margin. Obama’s victory over Romney in the state was by fewer than 75,000 votes and a 0.88 point margin.

Henry Crespo, president of the Democratic Black Caucus of Florida, told the Guardian he was also concerned, noting that Obama’s intensive ground strategy and black voter outreach paid dividends in 2012.

“We are lagging based on some numbers of black early vote participation, so going into these areas is so important,” he said. “The church has always been a bedrock of engaging people outside of other more traditional activities. That helps the word get out. That grandmother or that aunt or that granddad is a traditional churchgoer; if you engage that population they will ultimately engage their grandchildren, their nephews and so forth. It’s a strategy that’s a good approach given that we’re lacking in certain areas.”

At a symbolic night rally on Tuesday at the Rev Samuel Delevoe Memorial Park, named for one of Fort Lauderdale’s first black policemen who later became a civil rights leader, Clinton displayed no outward signs that she was concerned about the black vote, urging supporters: “For the next seven days focus on what is important. Don’t get distracted, don’t get diverted.”

For some black supporters in the crowd, however, the early voting figures were an alarm call. “It’s something we’re all concerned about, that’s why we’re all doing our best to get the word out, that we need to come together, stand together and move forward together,” said Lakisha Smith, 43, a customer service representative who attended with a small group of friends and colleagues.