MADISON, Wis. -- A poll released Wednesday shows that President Donald Trump's job approval in the key swing state of Wisconsin remains below 50 per cent, a troubling sign for Republicans 14 months before the 2020 election.

The Marquette University Law School poll also showed voter unease with the direction of the economy and tariffs, while there's strong support for gun control measures in the wake of mass shootings in Ohio and Texas.

The poll offers the latest glimpse into voter attitudes more than a year before the election. Wisconsin is one of four swing states -- Pennsylvania, Michigan and Florida are the others -- that could determine the outcome of the 2020 election because their electorates are so evenly divided. Trump carried Wisconsin by fewer than 23,000 votes in 2016, becoming the first Republican presidential candidate to win the state in 32 years.

The poll showed Trump's approval rating was 45 per cent among registered voters, while 53 per cent disapprove of the job he's been doing. That is nearly unchanged from Trump's 46 per cent job approval from the last Marquette poll five months ago.

For the first time, the poll asked voters about the large field of Democratic candidates running to take on Trump. Sen. Joe Biden came out the strongest, with 51 per cent support compared with 42 per cent for Trump in a head-to-head contest.

"It's a significant 9-point lead," pollster Charles Franklin said, noting the Delaware senator's high name recognition given his eight years as vice-president under President Barack Obama.

Vermont Sen. Bernie Sanders, who won the Democratic presidential primary in Wisconsin in 2016, had 48 per cent support compared with 44 per cent for Trump. No other Democratic candidate came out ahead of Trump.

Biden was atop the Democratic field with 28 per cent support, followed by Sanders at 20 per cent, Sen. Elizabeth Warren, of Massachusetts, at 17 per cent. No one else was in double digits.

Franklin cautioned that the poll measures only current voter attitudes and is not a predictor of what might happen in the election that's more than a year away.

"We are in the second inning of a ballgame," Franklin said.

The numbers are particularly likely to change given the high percentage of voters who said they didn't know enough about other Democratic candidates to form an opinion, Franklin said.

On the economy, more people -- 37 per cent -- said they thought it would get worse over the next 12 months compared with those -- 26 per cent -- who thought it would improve. Thirty-three per cent said they thought it would stay the same.

On tariffs, 46 per cent said they hurt the economy while 30 per cent said the help it.

Democrats have been blasting Trump's approach to trade and increasing tariffs, particularly on China. Democratic U.S. Rep. Ron Kind, who represents mostly rural western Wisconsin, said the tariffs have been particularly painful for farmers who were already struggling and going bankrupt at record rates in the state.

"I've never seen a president work harder to plunge our nation into a recession," Kind said Wednesday during a call with reporters.

The poll also looked at state issues, including how voters feel about the job being done by Democratic Gov. Tony Evers and the Republican-controlled Legislature. This is the first poll conducted since the Legislature passed, and Evers signed, the state budget in July.

Evers' job approval rating was 54 per cent, up from 47 per cent in April. The Legislature's approval rating was 52 per cent, up slightly from 50 per cent five months ago.

Evers has been calling on Republicans to pass a universal background check bill for gun purchases and a "red flag" law that would allow judges to take firearms away from people determined to be a risk to themselves or others.

The poll showed very strong support for both measures, with 80 per cent supporting background checks and 81 per cent favouring red flag laws.

Republicans have voiced opposition to the measures and Evers has threatened to call a special session for them to take it up. But even doing that wouldn't force lawmakers to act on the measures.

The poll of 800 registered voters was taken between Aug. 25 and Aug. 29. It has a margin of error of plus or minus 3.9 percentage points, but that grew to 5.3 per cent when asking only Democrats questions. Issues-related questions had a smaller sample size, resulting in an error rate of about 5.5 per cent.