Washington - President Barack Obama will campaign with presumptive Democratic White House nominee Hillary Clinton for the first time next Tuesday, as a new poll shows a tightening race with Republican Donald Trump.The Democratic pair is scheduled to visit Charlotte, in the swing state of North Carolina, where they will "discuss building on the progress we've made and their vision for an America that is stronger together", Clinton's campaign said in a statement on Wednesday.Their debut joint campaign appearance for the 2016 election had been scheduled for June 15 in Wisconsin, but was postponed due to the massacre in Orlando, Florida - the worst mass shooting in US history.Obama endorsed Clinton on June 9 after months of assiduously avoiding tipping the scales of the Democratic presidential primaries."I don't think there's ever been someone so qualified to hold this office," Obama said in a video message that day as he offered his full-throated endorsement of the former secretary of state, senator and first lady."I'm with her, I am fired up, and I cannot wait to get out there and campaign for Hillary," added Obama, who won a brutal, months-long Democratic primary battle against Clinton in 2008.North Carolina is one of about a dozen battleground states where November's election is expected to be decided.Obama won North Carolina by less than half a percentage point against Republican John McCain in 2008. Four years later, Obama lost it to Mitt Romney by two points.Bruising showdownThe Democrats' joint appearance the day after the July 4 Independence holiday comes with US Senator Bernie Sanders refusing to bow out of the nomination race, despite Clinton amassing the necessary number of delegates to clinch outright victory at next month's party nominating convention.But Clinton has moved on, turning the entirety of her effort towards what is expected to be a bruising showdown with Trump as she aims to make history as the nation's first female commander in chief.The presidential race is too close to call, with the brash billionaire narrowing the gap with Clinton, according to the latest Quinnipiac University national poll.Respondents put Clinton ahead of Trump just 42% to 40%, a narrowing from Clinton's four-point margin in the organisation's June 1 survey.Eighteen percent gave no answer or said they would vote for someone else, or said they would not vote.The poll is considerably closer than the 12-point Clinton advantage in an ABC News/Washington Post poll published on Sunday which highlighted Clinton capitalising on Trump's recent mis-steps.The Quinnipiac survey notably showed that 61% believe the 2016 election "has increased the level of hatred and prejudice" across the country.Of that group, two thirds blame the Trump campaign, with just 16% blaming Clinton's team."Voters find themselves in the middle of a mean-spirited, scorched earth campaign between two candidates they don't like. And they don't think either candidate would be a good president," said Quinnipiac poll assistant director Tim Malloy.Trump has made several incendiary statements during the campaign, including a call for banning Muslims from entering the United States, and describing Mexicans as rapists and criminals.Respondents said by 58% to 33% that Clinton is better prepared to be president than Trump.But they said Trump would be better at creating jobs (52-40%) and at handling Islamic State extremists (52-39%) than Clinton.Meanwhile, respected election data analyst and FiveThirtyEight.com founder Nate Silver, who correctly predicted 50 out of 50 states in the 2012 presidential election, on Wednesday said Clinton was a 75- or 80% favourite over Trump."She's taking a seven-point, maybe a 10-point lead into halftime," Silver told ABC's Good Morning America."There's a lot of football left to be played, but she's ahead in almost every poll, every swing state, every national poll," he said of Clinton.