Sen. Lindsey Graham, R-S.C., and Sen. Jeff Flake, R-Ariz. AP Photo/J. Scott Applewhite President Donald Trump, a Republican, publicly attacked two GOP senators in a series of tweets Thursday morning.

"Great to see that Dr. Kelli Ward is running against Flake Jeff Flake, who is WEAK on borders, crime and a non-factor in Senate. He's toxic!" Trump tweeted, taking a shot at the junior Arizona senator.

Flake is up for reelection in 2018 and is considered one of the more vulnerable GOP senators in the next election cycle. He recently released a book, "Conscience of a Conservative," that is critical of Trump.

An excerpt of the book in Politico described an executive branch "in chaos" and a president who has "seeming affection for strongmen and authoritarians."

One of Trump's top benefactors, Robert Mercer, reportedly donated $300,000 to a super PAC charged with defeating Flake in 2018. A recent poll found that Flake's popularity in Arizona took a big hit after he voted for the senate healthcare plan that was subsequently defeated.

The president also lashed out at Sen. Lindsey Graham of South Carolina, another frequent critic of his.

"Publicity seeking Lindsey Graham falsely stated that I said there is moral equivalency between the KKK, neo-Nazis & white supremacists and people like Ms. Heyer," Trump tweeted Thursday morning, referencing Heather Heyer, who died in Charlottesville on Saturday.

"Such a disgusting lie. He just can't forget his election trouncing. The people of South Carolina will remember!" Trump tweeted, referring to Graham.

Graham, who ran for the Republican presidential nomination in 2016 and has been critical of Trump, said in a statement Wednesday that Trump "took a step backward by again suggesting there is moral equivalency between the white supremacist neo-Nazis and KKK members who attended the Charlottesville rally and people like Ms. Heyer. I, along with many others, do not endorse this moral equivalency."

Graham was responding to Trump's statements on Tuesday, where the president stated there was "blame on both sides" for the violence in Charlottesville.

Graham is not up for reelection until 2020.