A day after remarks by Sen. Ted Cruz (R-TX) that were mildly critical of Donald Trump were leaked from a private New York fundraiser, the GOP frontrunner fired a verbal shot across the bow of his rising GOP challenger, possibly signaling a new chapter in the Republican race for President.

With Cruz rising in the polls - and even leading in one from Iowa this week - Trump made clear that if Cruz starts attacking, "he will fall like all others" who have tried to disloged the GOP leader.

For Republicans, the warning shot on Twitter was a familiar tactic from Trump, who has often used social media to jab at his opponents.

As for Cruz, he has gone out of his way in recent months to avoid criticizing Trump, admitting in a fundraiser that he hopes to pick up the supporters of both Trump and Ben Carson in the months ahead.

"I don't think either one of them is going to be our president," Cruz said according to a recording of his remarks obtained by the New York Times.

The possibility of Trump and Cruz mixing it up comes at a time when the Texas Senator has emerged as the main threat to Trump in Iowa, which votes in just over seven weeks.

The warning shot on Twitter did not go unnoticed by campaign observers:

Cruz has gone out of his way in recent months to avoid criticizing Trump, at times seemingly drafting off the GOP leader.

This week, Cruz was asked several times about Trump's controversial plan to bar Muslims from entering the United States; each time, Cruz quietly said he disagreed, but refused to elaborate.

Cruz, meanwhile, made clear he's not going to get in a shooting war with Trump.