Early voting in key battleground states showed the US presidential campaign on a knife-edge, with both camps insisting the closely watched data showed their candidate gaining momentum as the race entered its final weekend.

Although the ballots – some submitted by mail, but most cast in early-opening precincts – will not be tallied until election day Tuesday, Democratic voters were turning up in greater numbers than Republicans in Nevada and North Carolina, while Republicans were outperforming in Ohio and Iowa.

According to Michael McDonald, a professor at the University of Florida who runs the US Elections Project, 37m Americans had already voted by yesterday morning and as many as 40 per cent could cast their votes before Tuesday.

In 2012, total early voting was just 32.3m. Early indications showed increased turnout among Latinos, seen as a fillip to Hillary Clinton, and whites, where Donald Trump pulls support – giving both campaigns reason to trumpet the data.

But with polls showing Mrs Clinton ahead at the national level and facing tight races in many of bellwether states, early voting suggests Mr Trump will need to generate a decisive turnout on Tuesday to overcome the electoral college advantage held by the Democrats.

Mrs Clinton and Mr Trump have been campaigning in early voting states for much of the week, particularly Florida and North Carolina, both essential for Mr Trump.

Mr Trump is due to visit them this weekend, while Mrs Clinton is expected in south Florida today. The nastiness of the campaign continued yesterday, with Mr Trump insisting a Clinton victory would mean Washington being tied up with scandals, while Mrs Clinton returned to the theme that her rival was unfit.

“She will be under investigation for years; she will face trials,” Mr Trump said in New Hampshire about inquiries into Mrs Clinton’s email practices. At a campaign event in Pittsburgh, Mrs Clinton said: “Imagine how easy it would be for Donald Trump to feel insulted and start a real war – not just a Twitter war – at three in the morning.” Democrats have built a strong advantage in Nevada.

By Thursday evening, registered Democrats made up 42.3 per cent of the returned ballots and Republicans 36.9 per cent – a similar gap to 2012 when Barack Obama carried the state by seven points.

In North Carolina, the Democrats are also ahead in early voting by 42.7 per cent to 31.8 per cent, however Michael Bitzer, a political scientist at Cawtaba College, points out that Republicans were performing better at this stage than they were in 2012 when Mitt Romney narrowly won the state. Even in the early voting, Florida is extremely tight with Republicans casting 39.74 per cent of the votes as of yesterday and Democrats 39.71 per cent.

There was better news for the Republicans in Iowa, where the Democrats lead in early voting was well down on 2012 levels and in Ohio where they have seen a bigger increase in requests for early ballots than the Democrats and where there were indications of reduced early voting around Cleveland, a Democratic stronghold.

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