Graham is the highest GOP office holder so far to rule out voting for Trump.
Other Republicans, such as Sen. Ben Sasse (R-Neb.), have also said they won't support Trump in the fall, while Speaker Paul RyanPaul Davis RyanDon't let them fool you — Republicans love regulation, too Senate harassment bill runs into opposition from House The Hill's Morning Report — Sponsored by PhRMA — The art of walking away from the deal MORE (R-Wis.) said Thursday he couldn't yet support the controversial businessman.
“It’s hard to believe that in a nation of more than 300 million Americans Hillary ClintonHillary Diane Rodham ClintonElection fears recede for House Republicans To woo black voters in Georgia, Dems need to change their course of action Trump lawyer touts petition to stop 'soft coup' against Trump MORE and Donald Trump will be our choices for president," Graham said in a statement released by his Senate office. 

Graham said he'd "absolutely" not support Clinton, the overwhelming favorite for the Democratic nomination, saying she'd represent President Obama's "third term."

“I also cannot in good conscience support Donald Trump because I do not believe he is a reliable Republican conservative nor has he displayed the judgment and temperament to serve as commander in chief," said Graham, who ran for president against Trump during the GOP primary.

Graham said after the November election he'd work with the next president to "deal with the many challenges facing our nation." 

“I will enthusiastically support Republicans for other offices in South Carolina and throughout the country," he said, adding his focus was on supporting Republicans in the House and Senate. 
Graham also announced he doesn't plan to attend the GOP convention in July in Cleveland. Several prominent Republicans, such as Mitt Romney and Jeb Bush, have said they don't intend to go, either.
"I will help President TrumpDonald John TrumpTrump appears to confirm deal on Chinese firm ZTE Judge rejects Manafort's attempt to throw out some charges Dem: Trump’s policy of separating children, parents at border ‘would shock Jesus’ MORE or President Clinton, because they'll need all the help they can get," Graham added during an interview with CNN's Dana Bash on Friday afternoon. 
Graham admitted his position may be unpopular, saying "some of my best friends" will support Trump. 
"I just can't go there," he added.
"Maybe I'm the outlier here, probably am. There will be Democrats that can't support Hillary Clinton," Graham said. 
The South Carolinas senator laughed while discussing the potential for Trump and Ryan working together in the near future after the pair exchanged attacks this week. 
"I hope he and Paul can find a common agenda," Graham said. "We'll see what happens."
However, Graham said Trump's detractors in the party should not not mount a third-party candidacy, as some have suggested doing to derail Trump.
"I would advise people not to go down that road," Graham said. 
He slammed Trump's attempts so far to expand the party's base.
"Eating a taco is probably not going to fix the problems we have with Hispanics," he said.
The senator feuded repeatedly with Trump before and after leaving the GOP race in December. Trump last year gave out Graham's cell phone number after the senator called the businessman a "jackass."

The South Carolina senator said during a speech in Beverly Hills, Calif., this week that the GOP had "lost its way" with Trump, whose foreign policy views he harshly criticized.

— This story was updated at 2:38 p.m.