The Ted Cruz versus Donald Trump battle has made its way to South Carolina.
If Cruz "doesn't clean up his act, stop cheating, & doing negative ads" then Trump has "standing to sue him for not being a natural born citizen," the Republican presidential front-runner tweeted on Friday, reiterating the warning for the second day in a row.
The tweet dredges up an attack leveled by Trump before the Iowa caucuses. He has been publicly speculating that Cruz is not actually eligible to run for president since he was born in Canada and, until recently, held dual American and Canadian citizenship. The Constitution requires a president to be a "natural born citizen." And while Cruz was born an American citizen given his mother's nationality, some scholars Trump cites have said that the issue is far from clear.
 Trump Threatens to Sue Cruz over Birther Questions 2:20
Trump and Cruz, both with one early-state win under their belt, are vying with each other for a second victory in the Palmetto State. But the state, which holds its Republican primary vote on February 20, is well known as a venue for dirty tactics employed by candidates who campaign there — and Trump is making no secret of calling Cruz out for his.
On Thursday evening, after a 10,000 person rally in Baton Rouge, Louisiana, Trump tweeted about a "lying Cruz" and pushed back against allegations from the Texas senator that Trump and Rubio "are w/ Obama on gay marriage." Trump followed up: "Cruz is the worst liar, crazy or very dishonest. Perhaps all 3?"
Friday morning, Trump went further, questioning "how can Ted Cruz be an Evangelical Christian when he lies so much and is so dishonest?"
The two Republicans began the election cycle as friends, participating in a sort of political détente and refusing to attack the other. That peace period was broken before the Iowa caucus as Cruz inched past Trump in many polls.