Washington: Indiana voters cast their ballots in key US presidential primaries Tuesday with Republican Donald Trump hoping to knock out his rivals and Hillary Clinton seeking to further cement her status as the Democrats' presumptive pick.

The latest contest in the 2016 race for the White House is seen as a day of reckoning for the movement to "stop Trump" led by his closest rival Ted Cruz.

But the billionaire real estate mogul -- who has thus far defied all political logic to lead the Republican race -- looked set to deliver a death blow to Cruz with a new NBC poll giving Trump a 15-point advantage over the conservative Texas senator.

"I don't think he's got the temperament to be president" Trump said of Cruz in an interview with Fox News after polls opened.

"People are tired with what's happening with these politicians and they're just tired of seeing our country get ripped off."

Clinton and her rival Bernie Sanders were locked in a closer race in Indiana with the former secretary of state in the lead by just under seven percentage points according to the RealClearPolitics poll average.

The 68-year-old Clinton is already so far ahead overall that Sanders' only hope now lies in the unlikely scenario of her failing to win a majority of delegates in the primaries in which case her nomination could be contested at a Democratic convention in July.

Cruz was counting on a similar scenario with Indiana acting as a Trump firewall blocking him from receiving the 1237 delegates necessary to secure the nomination at the Republican convention in Cleveland in July.

Mathematically eliminated from winning outright Cruz's goal is to snatch victory on a second ballot when most delegates become free to vote for whomever they choose -- but which will only be held if Trump falls short of a majority in round one.

- 'Arrogant buffoon' -

With momentum favoring the 69-year-old Trump who has clinched the last six contests the two rivals attacked each other on an unlikely front Tuesday -- a tabloid report linking Cruz's father Rafael to John F. Kennedy assassin Lee Harvey Oswald.

Trump invoked the recent National Enquirer story in his interview with Fox News.

"This is just kooky" an irate Cruz shot back while stumping in Evansville Indiana branding Trump a "pathological liar."

"The man is utterly a moron" he said.

"A caricature of a braggadocious arrogant buffoon who builds giant casinos with giant pictures of him everywhere he looks."

Should Cruz fall short Tuesday even his supporters see an extremely steep road ahead.

Indiana's primary awards 30 delegates to the state's winner. The remaining 27 are awarded three each to the winner of the state's nine congressional districts.

If Trump sweeps the state "it could be over" former Cruz spokesman Rick Tyler acknowledged on MSNBC.

But if Cruz pulls off an upset Tyler said he would be strongly positioned in California which votes on June 7 on the final day of the Republican race.

Trump has so far amassed 1002 delegates according to CNN's tally. He needs just under half of the 502 in play in the remaining 10 contests to lock in the nomination. Cruz is at 572 delegates.

The map currently favors the billionaire who is polling well ahead in the largest states yet to vote -- California and New Jersey.

- 'Let's focus on Hillary' -

Clinton needs only 21 percent of remaining Democratic delegates to win her party's nomination.

But Sanders a self-proclaimed democratic socialist senator representing Vermont wasn't throwing in the towel.

Courting the union vote in Indiana where manufacturing has taken a hit Sanders sought to focus on trade.

"We need a trade policy which works for the middle class and working families and not just for the CEOs of large corporations" he said outside a diner in Indianapolis.

"Secretary Clinton has supported virtually every one of these disastrous trade agreements and that is an area of strong disagreement that the voters of Indiana and America will have to consider."

Either candidate needs 2383 delegates for victory. Currently Clinton has 2179 including 513 superdelegates while Sanders has 1400 including 41 superdelegates according to CNN's tally.

A confident Trump was already looking beyond Tuesday to a general election matchup with Clinton.

"Please let's focus on Hillary" he said on the eve of the vote.

Clinton also had her sights set on November.

"I am focused on moving into the general election" she said Tuesday in West Virginia.

"That's where we have to be because we are going to have a tough campaign against a candidate who'll literally say or do anything."

AFP