Democrats have defeated Republicans in key U.S. state races that will be closely scrutinized for trends relevant to the 2020 presidential election and President Donald Trump's chances of a second term.

The Democrats declared victory on Tuesday night in the election for the governor's mansion in Kentucky and took control of Virginia's state legislature, while the Republicans held on to win the Mississippi gubernatorial contest.

Beshear declares victory in Kentucky

Democratic State Attorney General Andy Beshear looked set for a win over incumbent Republican Matt Bevin in Kentucky, despite Trump hosting a mass rally in the state on Monday.

Beshear declared victory late on Tuesday, though Bevin is yet to concede in a state where Democrats hope to oust Republican Senate Majority Leader Mitch McConnell in 2020.

Kentucky Secretary of State Alison Lundergan Grimes told CNN that while there was a possibility of a recanvas or a recount if Bevin requested it, she was confident 41-year-old Beshear had flipped the governor's mansion.

Trump had told supporters in Lexington on Monday that Kentuckians need to re-elect Bevin or pundits will say: "Trump suffered the greatest defeat in the history of the world... You can't let that happen to me."

The attempt to make the election about Trump, who won the state by nearly 30 points in the 2016 presidential election, was not enough to swing the result Bevin's way. 

Beshear led by around half a percentage point, overturning Bevin's nine-point victory in 2015. Initial data suggested that urban and suburban areas came out strongly for the Democrat, who also picked up a sprinkling of rural counties.

Democrats take Virginia legislature

Democrats flipped both chambers of the Virginia legislature from the Republicans, giving the party complete control of the state government for the first time in decades and in a strong position ahead of the 2020 election.

Trump has avoided campaigning in Virginia, where Democrats found success in suburban swing districts in last year's congressional polls.

In Mississippi, another state Trump won convincingly in 2016 and campaigned in ahead of Tuesday's election, Republican Lieutenant Governor Tate Reeves beat Attorney General Jim Hood by around six points.

New Jersey also held legislative elections on Tuesday.

What does it mean for 2020?

Victory in red Kentucky combined with complete control in Virgina is big for the Democrats and a blow for the Republicans.

That Trump had to hold last-minute rallies in Kentucky and Mississippi, traditional Republican strongholds, is not a good sign for the party heading into 2020.

But while Bevin and Trump sought to nationalize the race, the incumbent governor had many local problems.

He was a deeply polarizing candidate – opinion polls suggested the 52-year-old was one of the least popular governors in the country – and Republicans lower down the ballot performed well in Kentucky.

On the issues, Beshear stayed away from impeachment, the Trump presidency and concentrated on healthcare. Former Ohio governor John Kasich told CNN this was the type of moderate approach Democratic hopefuls should try to replicate in 2020.

Beshear was also boosted by name recognition: His father was a previous governor of the state.

A key trend of the 2018 midterm elections looks to have been continued in Kentucky and Virginia, with suburban voters moving away from the Republicans to the Democrats – a major danger sign for Trump's party.