Sen. Lindsey GrahamLindsey Olin GrahamTop Louisiana health official rips Cassidy over ObamaCare repeal bill Senate Dems hold floor talk-a-thon against latest ObamaCare repeal bill Overnight Defense: Senate passes 0B defense bill | 3,000 US troops heading to Afghanistan | Two more Navy officials fired over ship collisions MORE (R-S.C.) says he is "all in" for President Trump despite his contentious relationship with Trump during the election.

"[Trump] calls, I am really pleased [with] what he has done to rebuild our military, to restore our standing," Graham told "Fox & Friends" on Wednesday.

"I want to cut taxes and reform healthcare. He's got the best national security team I've seen since I've been in government. I am all in. Keep it up, Donald. I am sure you are watching. Don't let these guys talk you out of being tough, because you need to be tough," he added.

Graham has been a consistent advocate for bold positions on Russia and North Korea and has frequently encouraged Trump to adopt a firmer foreign policy.

During and after the 2016 election campaign, the pair often clashed over topics related to Russia, foreign policy and immigration.

"Lindsey Graham, Lindsey Graham, I've been competing with him for a long time. He's going to crack that 1 percent barrier one day," Trump said in January after Graham proposed tougher sanctions on Moscow.

Trump and Graham clashed early in the GOP presidential primaries when they were both candidates, with Trump publicly revealing Graham's personal cellphone number after facing criticism from the senator.

After dropping out, Graham urged voters to support Sen. Ted CruzRafael (Ted) Edward CruzSenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill Overnight Finance: CBO to release limited analysis of ObamaCare repeal bill | DOJ investigates Equifax stock sales | House weighs tougher rules for banks dealing with North Korea GOP state lawmakers meet to plan possible constitutional convention MORE (R-Texas) in the primaries because he "made the best case" that he can beat Trump.

"Failed presidential candidate Lindsey Graham should respect me," Trump tweeted in response. "I destroyed his run, brought him from 7% to 0% when he got out. Now nasty!"