In a boost for Democrats ahead of the midterms, two swing states have seen extraordinary voter turnout in the party’s primary elections.

Minnesota and Wisconsin saw dramatic increases in participation as both Democrat and Republican voters picked their candidates for Congress, the senate and governor.

Democrats are hoping to replicate successes seen around the nation over the past 18 months in local and state special elections, where high levels of enthusiasm have propelled them to victories in a number of races, including deep red Alabama’s senate seat.

In Minnesota, total primary turnout at more than 875,000 surpassed a 20-year high, with Democrat votes outnumbering Republican ballots by more than 250,000, according to unofficial state figures.

Wisconsin saw a similarly high level of voter turnout at around 950,000 ballots cast – numbers not seen since 2002 - with Democrats again outvoting Republicans by more than 80,000.

Turnout was strong in Democratic strongholds such as Madison, the state capital, and the largest city, Milwaukee, but also in Republican suburbs around Milwaukee.

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“If the election were held today, it’s highly unlikely that Trump would carry Wisconsin again,” said Charlie Sykes, a conservative commentator and longtime Trump critic.

Donald Trump became the first Republican to win Wisconsin in 32 years when he edged out Hillary Clinton there in 2016.

The US president has claimed the midterm elections could see a “red wave”, with Republicans sweeping to victories across the country and strengthening the party’s hold on both the house and senate.

Did someone give Minnesota Republicans the wrong primary date, because Dem turnout is better than two-to-one Jim Kessler, coBlock quote caption

Democrats hope they can seize enough seats in Congress to take control of both the house and senate, and so deprive Mr Trump of the ability to push through much of his far-right agenda.

And similar disparities in voting between the two parties in November's midterms would indeed prove disastrous for Mr Trump and the Republican Party.

Jim Kessler, co-founder of centre-left think tank Third Way, said Minnesota turnout was a “bad sign” for Republicans going into the midterms.

“Did someone give Minnesota Republicans the wrong primary date, because Dem turnout is better than two-to-one,” he added.

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Cliston Brown, a US political analyst, said primary turnout was “often telling” regarding turnout in general elections.

“One of the little-noticed facts in 2016 was the Republican primary turnout vis-a-vis the Democratic turnout,” he wrote on Twitter. “This year the shoe is very clearly on the other foot.”

However, David Canon, professor at the University of Wisconsin-Madison, said not too much could be read into Tuesday’s turnout in Wisconsin, given several hot contests in the state.