"Ted Cruz is a total liar," said Donald Trump, the real- estate tycoon-turned Republican presidential aspirant.
Trump, who was trailing behind the Texas Senator, has now taken a lead over Cruz as per the latest polls.
The 69-year-old billionaire, now considered a Republican favourite, hoped that a large voter turnout in the Iowa Caucus would seal his victory in the first primary.
"The more people that get out the better I do. And I think we're going to have a big crowd," he told ABC News as he continued to slam Cruz, who is now seen as the only candidate standing between Trump and victory in the primaries.
Replying to Trump's barrage against him, Cruz told Fox News: "Donald looks very rattled there."
"Donald is engaging in daily insults...There's a reason he's engaging in personal insults. He doesn't want to talk about the issues. He doesn't want to talk about substance," Cruz said as he made rounds of Sunday talk shows.
Later, Cruz claimed on CNN that he was "positioned to do very well" in Iowa, which is a life and death situation for his presidential candidature.
Clinton, 68, in an interview to ABC conceded that the race with Bernie Sanders was competitive.
"That's why I hope everybody who has decided to caucus for me will be sure to come out on Monday night," she said.
Sanders, who is giving a tough fight to Clinton as per latest poll figures, argued that voters' turnout now holds the key to his election strategy.
"I think, if the turnout is high, we have got a real shot to win this," the 74-year-old told CNN.
"That's the difference. The American people want us to move in a very different direction. They want government to represent all of us, not just the wealthy. That's why our campaign is doing well. That's why I think we can win the general election," he said.
