By Arun Kumar

Washington, Feb 13 (IANS): Ahead of the next round of nominating contests, the US presidential race is turning testy with rival candidates of both parties attacking each other with no holds barred.

Republican frontrunner Donald Trump on Friday threatened to sue rival Iowa caucus winner Ted Cruz, for "not being a natural born citizen" if the Texas senator "doesn't clean up his act" and stop running negative ads against him.

Trump has previously argued that if Cruz won the Republican nomination, Democrats would argue that the Canada-born candidate was ineligible for the presidency.

Trump also questioned the sincerity of Cruz's faith, accusing his opponent of being "so dishonest."

"How can Ted Cruz be an Evangelical Christian when he lies so much and is so dishonest?" he tweeted.

"There is more than a little irony in Donald accusing anyone of being nasty given the amazing torrent of insults and obscenities and vulgarities that come out of his mouth," Cruz told reporters in South Carolina on Friday.

"Being attacked by Donald, it is always colourful. I will give him this: he's not boring."

Cruz's campaign also pulled down an attack ad on Florida senator Marco Rubio after it was revealed that one of the actors Amy Lindsay had performed in soft core porn films.

Lindsay had merely responded to an open casting call and blamed the company that recruited the actors for not properly vetting those who appeared, Cruz said.

"It happened that one of the actresses who was there had a more colourful film history than we were aware," he said. "We would not have cast her had we known of that history."

"I have no ill will towards Ted Cruz right now. He's got a job to do," Lindsay told CNN. "And I'm a middle class working girl and I had a job to do."

Meanwhile, after Thursday night's sharp exchanges during the Democratic debate, the battle between Democrats Hillary Clinton and Bernie Sanders is also turning testy.

Representatives from both campaigns have accused the other side of mudslinging and negative campaigning.

Sanders' campaign manager Jeff Weaver accused Clinton of "flinging personal mud" at Sanders.

"The mudslinging seemed really out of place," Weaver told CNN. "It seemed very much like what you see in a Republican debate."

Jennifer Palmieri, Clinton's communications director, in turn accused Sanders of getting "increasingly personal and negative."

"He's had a lot of innuendo about how she took donations [from Wall Street] and how that must mean it's affecting her views -- without any evidence."