Democrat Hillary Clinton, turning her attention to an increasingly likely match-up against Republican Donald Trump, has sought to shore up union support by visiting coal and steel workers in the economically struggling Appalachian region.

While the Republican presidential candidates focus on Tuesday's primary contest in Indiana, Clinton will meet the head of a local steel workers' union and retired mine workers in Kentucky, West Virginia and Ohio.

Trump's pro-coal, anti-trade message has resonated with voters frustrated over job losses.

Clinton has pledged more than $US30 billion to help regions that depend on coal, but her promise was overshadowed when she said in March that the country would "put a lot of coal miners and coal companies out of business".

Parts of Appalachia, a region that spans multiple states across the eastern United States, have struggled with poverty and losses of jobs.

West Virginia's unemployment rate of 6.5 per cent in March was well above the national rate of five per cent, according to Labor Department data.

Ohio's unemployment rate was 5.1 per cent, while the figure in Kentucky was 5.6 per cent.

Clinton has a large lead over US Senator Bernie Sanders for the Democratic nomination, and she is making early moves like this week's trip to try to siphon support from Trump, who says his outsider campaign will succeed with struggling voters in the November election.

Trump will take a leap toward winning the Republican nomination if he comes out ahead in Tuesday's Indiana primary.

His success in the race for the White House may well ride on the support of Republican evangelicals.

On Monday, the New York billionaire criticised a trade deal signed by former president Bill Clinton, and threatened tariffs on goods from companies that move out of the United States in search of cheaper labour.

"People look for a job and they have to quit after four, five months," Trump said on CNN.

"I think that a lot of these people are going to join my campaign. I think a lot of the Bernie Sanders young people are going to join my campaign."