[WASHINGTON] Hillary Clinton sought to halt the momentum of Democratic presidential rival Bernie Sanders on Tuesday in two key primaries, but the race in Kentucky was too close to call shortly after polls closed.

Mrs Clinton has a commanding lead in the all-important delegate count and is marching toward the nomination despite a string of recent losses.

A pair of victories Tuesday would halt her slide and help reverse the narrative that her campaign is showing significant weakness ahead of an almost certain November showdown between Mrs Clinton and Donald Trump, the Republican Party's presumptive nominee.

Though Mrs Clinton holds a slim poll lead, Mr Sanders was gunning for victory in Kentucky, building on his win last week in neighboring West Virginia as he battles to keep his long-shot nomination bid alive.

The self-declared democratic socialist was leading 47.2 per cent to 46.1 per cent, with about 40 per cent of precincts reporting.

West Virginia and Kentucky are linked to coal, as is much of Appalachia - the largely white, long-struggling eastern US region where many feel they have been left behind in the lukewarm recovery from the 2007-2008 financial crisis.

Oregon in the Pacific northwest conducted its Democratic and Republican primaries Tuesday. Polls there close at 8:00 pm (0300 GMT Wednesday).

"We need your help today to win in Oregon and Kentucky," Mr Sanders implored his 2.2 million Twitter followers, urging them to man phone banks and call voters.

Limited Oregon polling has indicated Mrs Clinton is ahead. In Kentucky, only Democrats were voting for presidential candidates; Republicans have already held a caucus here.

Mrs Clinton sees Kentucky as an opportunity to appeal to working-class white men - a demographic where the former secretary of state has lagged behind both the celebrity billionaire Mr Trump and Mr Sanders.

John Spenlau, 28, speaking to AFP outside a voting station in rainy suburban Louisville, said he voted for Sanders because he represented the best hope for "continued change" and the fight against income inequality, among other problems.

"Hillary would be a more stable candidate but I think that Bernie continues to push the envelope, towards a few more of the social programs that I believe in," Mr Spenlau said.

Mrs Clinton defeated Barack Obama in Kentucky's primary in 2008, and her husband Bill Clinton was the last Democrat to carry the Bluegrass State in a general election.

On Sunday, playing up her husband's popularity, the former first lady appeared to indicate Bill would play a role in her administration if she were elected, promising to put him "in charge of revitalising the economy." She later said she would not give him a cabinet position.

As voting got underway, Mr Trump fired off a series of nasty tweets at Mrs Clinton.

"Crooked Hillary can't close the deal with Bernie Sanders. Will be another bad day for her!" Mr Trump wrote.

The Clintons have struggled to contain the damage from comments Hillary made in March, when she said she vowed to put many coal miners "out of business" in the transition to cleaner energy.

In recent appearances she has struck a more inclusive tone. "We need to take coal country, coal miners and their families with us and not leave them behind," Mrs Clinton told supporters Monday at a rally in Bowling Green, Kentucky.

Mrs Clinton, 68, made three stops in Kentucky on Sunday and four more Monday, shaking hands, taking selfies, offering hugs - even chatting with Trump supporters at a smoke-filled diner in the southwestern city of Paducah.

"I will not vote for you. I will never vote for you," disabled veteran Dianna Dooley, 66, told her.

Mrs Clinton kept her composure, saying: "That's OK. You vote for whoever you want." Mr Sanders, 74, has invested time in Kentucky as well, campaigning in Paducah on Sunday and Bowling Green on Monday.

But with the Democratic nomination in sight, Mrs Clinton is looking beyond the showdown with Mr Sanders to position herself for a bruising campaign battle against Mr Trump.

At a rally in Hopkinsville, Mrs Clinton pummeled the "risky and dangerous" Trump, suggesting he is unqualified to handle tough foreign policy decisions.

A main "super PAC" supporting Mrs Clinton on Monday unveiled a new television ad that skewers Trump for his treatment of women.

The ad, which will air across the crucial swing states of Ohio, Florida, Virginia and Nevada, shows women mouthing Mr Trump's own words, including "you could see there was blood coming out of her wherever," a comment Mr Trump made last year in reference to a debate moderator who asked him tough questions.

Mr Trump shot back Tuesday with criticism that could foreshadow the tone of the coming election battle.

"Amazing that Crooked Hillary can do a hit ad on me concerning women when her husband was the WORST abuser of woman in US political history," Mr Trump tweeted.

AFP