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.
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.
Mr Cruz attacked Mr Trump on Tuesday (local time), calling the billionaire businessman "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 1237 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 was eager to 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.
"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 co-ordinate their strategies against Mr Trump, a move which prompted Mr Trump to label them "desperate".