Democrats have opened a six-point lead over Republicans ahead of the midterm elections, according to a new poll.

As the US gears up for November’s ballot, which will see the two parties battle for control of congress, a fresh YouGov survey suggests 44 per cent of voters would opt for a Democrat if the election was held today.

That compares to 38 per cent who would vote Republican, and 14 per cent of respondents who remain undecided.

Billed as the first big referendum on Donald Trump’s first two years in office, the midterms have the potential to change the country’s political landscape.

Democrats have consistently led in the polls since the election of Mr Trump, though the gap appears to have swung wildly in recent months.

In January, Democrats led Republicans in a generic ballot by a huge 12 per cent. By April the lead was down to just 4 per cent, according to an ABC News and Washington Post poll.

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Currently, men favour the Republican party over the Democrats by one percentage point, whereas women prefer the Democratic party by a margin of 11 per cent.

The GOP holds an 8 per cent lead over Democrats among white voters, but the tables are turned dramatically among ethnic minorities.

Four out of five black people would vote Democrat, the poll suggests, compared to just 10 per cent who would opt for a Republican. Hispanics also favour Democrats by a margin of more than two-to-one.

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The YouGov poll randomly selected 1,500 US adults, more than 1,200 of whom are registered voters. The organisation said the survey has a margin for error of 3 per cent.

Currently, Mr Trump’s Republican Party controls both the House of Representatives and the Senate. In the House, 435 seats are up for election in the forthcoming midterms. The Republicans in that chamber currently hold 238 seats. The Democrats have 192.