[Arizona Sen. Jeff] Flake told Trump that he wants to be able to support him — “I’m not part of the Never Trump movement,” the senator said — but that he remains uncomfortable backing him, the officials said.

Trump said at the meeting that he has yet to attack Flake hard, but threatened to begin doing so. ...

... Trump also called out Sen. Mark Kirk (R-Ill.), who withdrew his endorsement of Trump last month citing the business mogul’s racially-based attacks on a federal judge, and said he did not approve of the senator’s action, said the officials.

Characterizing Kirk as a loser, Trump vowed that he would carry Illinois in the general election even though the state traditionally has been solidly Democratic in presidential contests. Kirk did not attend the meeting with Trump.

Trump also singled out Sen. Ben Sasse (R-Neb.), who has refused to support Trump and has emerged as perhaps the most vocal advocate for a third-party candidate. Sasse declined to speak with reporters as he left the meeting.