Sen. Lindsey GrahamLindsey Olin GrahamDHS chief takes heat over Trump furor Overnight Defense: GOP chair blames Dems for defense budget holdup | FDA, Pentagon to speed approval of battlefield drugs | Mattis calls North Korea situation 'sobering' Bipartisan group to introduce DACA bill in House 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 CruzWith religious liberty memo, Trump made America free to be faithful again Interstate compacts aren't the right way to fix occupational licensing laws Texas Dem: ‘I don’t know what to believe’ about what Trump wants for wall 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!"