US election 2016: Trump-Cruz row precedes Indiana primary
Republican presidential hopeful Ted Cruz has warned that America would "plunge into an abyss" if it elected Donald Trump.
He spoke after Mr Trump suggested Mr Cruz's father was connected to the man who killed President John F Kennedy.
The New York tycoon is poised to deliver a crushing blow to Mr Cruz as Indiana votes in the latest primary.
Polls have closed in some parts of the state but the last polls will not close until 1900EDT (2300GMT).
Mr Cruz's advisers had targeted Indiana as the senator's best hope of halting Mr Trump's march to the nomination.
However, polls show Mr Trump with a sizeable lead in the mid-western state.
The verbal attacks reached a new intensity on Tuesday, as Mr Cruz attacked the billionaire businessman as "totally amoral", "a pathological liar" and "a serial philanderer".
Responding, Mr Trump said "Ted Cruz is a desperate candidate trying to save his failing campaign.
"It is no surprise he has resorted to his usual tactics of over-the-top rhetoric that nobody believes."
Mr Cruz and fellow candidate John Kasich are hoping to force a contested convention where party officials, not voters, choose the nominee.
If Mr Trump wins Indiana, the New York businessman will probably reach the required 1,237 delegates to secure the nomination and avoid such a scenario.
Meanwhile in the Democratic battle, polls show Hillary Clinton and Bernie Sanders locked in a tight race in Indiana.
However, a Sanders win in Indiana would do little to erase Mrs Clinton's commanding lead.
The Clinton campaign has shifted its focus to other states, opting not to actively campaign or spend money in Indiana.
Mr Trump told supporters on Monday that he is eager turn his attention to the general election.
"You know if we win it's over, and then I can focus," he said at a campaign rally in the state.
Mr Cruz cannot get enough delegates to win the nomination outright, but hopes he can still deprive Mr Trump of a majority.
He has thrown considerable resources into the state in recent days.
At the scene: Anthony Zurcher, BBC North America reporter
Ted Cruz's father is tied to the Kennedy assassination. Donald Trump is a pathological liar, a narcissist and a serial philanderer. Mr Cruz is unhinged and desperate.
That is just a taste of the accusations that have been flying between the two top Republican presidential hopefuls on the day primary voters in Indiana head to the polls.
If recent opinion surveys are any indication, Mr Trump is poised to deliver a mortal blow to the Cruz campaign in a state the Texan had once described as "favourable terrain". Mr Cruz has thrown everything he can at his opponent - brokering a deal to clear out rival candidate John Kasich, breaking with tradition and naming his vice-presidential running mate early, even engaging Trump supporters in a street-side debate. None of it has worked.
Mr Cruz ran an efficient, tactically sound campaign that allowed him to survive long after most of his competitors had faltered. But now, facing Mr Trump in a head-to-head showdown with everything at stake, his best efforts may prove to be far from enough.
"Indiana is in a position to decide the direction of this race, not just for Indiana, not just for the Republican Party, but for the entire country," he told supporters in the state.
"I think... the country is depending on Indiana to pull us back from this cliff."
The third Republican candidate, Mr Kasich, is no longer campaigning in Indiana.
He and Mr Cruz reached a deal last month to coordinate their strategies against Mr Trump, a move which prompted Mr Trump to label them "desperate".