In an op-ed for Syracuse.com
, Hanna writes that Trump is "deeply flawed in endless ways" and "unrepentant in all things" before declaring: "While I disagree with her on many issues, I will vote for Mrs. Clinton."
While a number of Congressional Republicans have openly repudiated Trump and said they will not vote for him -- including Sen. Lindsey Graham (R-SC), Sen. Jeff Flake (R-AZ), and Sen. Ben Sasse (R-NE) -- Hanna, a resolute #NeverTrump-er, is the first to go as far as crossing party lines in his opposition to the GOP presidential nominee.
"Months ago I publicly said I could never support Trump," Hanna wrote, calling him "profoundly offensive," "narcissistic," and "a world-class panderer."
"I do not expect perfection, but I do require more than the embodiment of at least a short list of the seven deadly sins," he added.
Hanna wrote that he has "long held the belief that the Republican Party is becoming increasingly less capable of nominating a person who is electable as president," suggesting that Hispanics, women, and the LGBT communities are groups that the GOP has grown increasingly out-of-step with.
Turning to Clinton, he explained that "Mrs. Clinton has issues that depending on where one stands can be viewed as great or small. But she stands and has stood for causes bigger than herself for a lifetime. That matters."
"While I disagree with her on many issues," he said, "I will vote for Mrs. Clinton."
Also on Tuesday, amid fallout from Trump's controversial feud with the family of a slain Muslim U.S. soldier, another Republican House member -- Rep. Charlie Dent (R-PA) -- announced he "no plans" to vote for Trump.
"That's where I am," Dent said on MSNBC's "Morning Joe." "I'm like millions of Americans right now that I have to be persuaded, and I just haven't been."
Dent, who supported Ohio Gov. John Kasich during the primaries, said that Trump's "incendiary comments" combined with his "lack of policy specifics and the contradictory nature of those policies" have caused him "a great deal of concern."
Dent added that Rep. Richard Hanna (R-NY) called him Sunday and said he was most likely going to support Hillary Clinton. Dent described Hanna as a "good friend" and a "rock solid guy."
CNN's Rachel Chason contributed to this report.