Sen. Lindsey Graham Lindsey Olin GrahamThe Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Senate Democrats' campaign arm announces seven-figure investment to boost Graham challenger Graham: Comey to testify about FBI's Russia probe, Mueller declined invitation 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.

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"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 Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats 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!"