Joe Scarborough delivered a scathing assessment of Donald Trump on Thursday, laying into the candidate for picking petty fights with Republican leaders who haven't supported him.

The MSNBC host criticized Trump for denouncing some of his rivals for the GOP presidential nomination as "sore losers."

“The stupidity is breathtaking. Here you have a guy that delivers a speech that can actually convert Democrats in Ohio and Pennsylvania and across the Rust Belt … because he’s talking about people. He’s talking about other people’s jobs. He’s talking about bringing jobs back to America," said Scarborough, speaking on his show "Morning Joe." "Then he goes off on GOP candidates. They’re not going to endorse him now."

Earlier, Scarborough warned that Trump could suffer a landslide loss to former Secretary of State Hillary Clinton in November.

"There's got to be consistency every single day," said Scarborough. "If he doesn’t have the discipline to do that every day, Hillary Clinton is going to blow him out."

The MSNBC host's criticism was prompted by remarks that Trump delivered in Bangor, Maine, on Wednesday. Without naming anyone specifically, the presumptive GOP nominee complained about former rivals who have not endorsed him, even though they'd once agreed to support whoever emerged from the Republican field.

"They broke their word. In my opinion, they should never be allowed to run for public office again because what they did is disgraceful," Trump said.

Republicans who ran for president this year but haven't endorsed Trump include Texas Sen. Ted Cruz, former Florida Gov. Jeb Bush and Ohio Gov. John Kasich.

To Scarborough, focusing on the defeated candidates is pointless.

"Who cares? Who cares if Jeb Bush endorses him? Who cares if John Kasich endorses him?" Scarborough said. "Is he capable of being disciplined and moving on or not?

Early in the campaign, Scarborough showed a more chummy side to Trump, who made frequent appearances on "Morning Joe." But since then, he's become more critical. On Wednesday, Scarborough said that Trump had been "acting like a bigot."

Editor's note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims -- 1.6 billion members of an entire religion -- from entering the U.S.