He criticized Trump's rhetoric during a recent speech, stating it raised concerns about his fitness for the presidency.
Cassidy declined to endorse Trump for another term in the White House.
Despite his criticisms, Cassidy did not throw his support behind Trump's potential rival, Joe Biden.
Cassidy expressed disagreement with Trump's comments about pardoning individuals involved in the January 6, 2021, Capitol attack.
He pledged to vote for a Republican candidate in the upcoming presidential election but did not mention Trump by name.
Cassidy had previously called for Trump to drop out of the 2024 Republican primary race.
In January 2021, Cassidy voted to convict Trump on one article of impeachment related to inciting an insurrection.