Donald Trump is constantly using this insult to diminish Marco Rubio

Donald Trump has a new go-to insult aimed at rival Marco Rubio that seems designed to diminish him.

In tweets and in his campaign stump speeches, Trump has been repeatedly referring to the Florida senator as "Little Marco Rubio" and calling him a "lightweight."

On Sunday, for example, he tweeted about "little" Rubio three times in a row:

He also called Rubio a "frightened little puppy" at a rally on Friday and tweeted that same day that he looked "like a little boy" on stage. On Sunday, he repeatedly referred to "Little Marco Rubio" during a campaign rally in Madison, Alabama.

"He can't be elected dog-catcher in Florida," Trump said. "I call him, 'Little Marco.'"

Trump has focused on Rubio after spending weeks attacking Ted Cruz, the Texas senator who is also running for the Republican presidential nomination. Rubio went after Trump at the Republican debate on Thursday, which seems to have sparked Trump's new line of attacks.

Trump has previously used this strategy of repeating insults with former Florida Gov. Jeb Bush, who dropped out of the race for the Republican nomination after a disappointing showing in South Carolina.

Rubio acknowledged Trump's nickname for him at a rally on Sunday, firing back by saying that Trump has small hands.

"He's always calling me, 'Little Marco,'" Rubio said. "And I'll admit ... he's taller than me, he's like 6-2. Which is why I don't understand why his hands are the size of someone who's 5-2."

The two rivals spent the weekend bashing each other at various events, leading some to question whether the rhetoric will have a long-term effect on the Republican Party.