PHOENIX – In a farewell to Arizona, outgoing Republican U.S. Sen. Jeff Flake warned against the current direction of his own party and the growing split between Americans.

Once considered a rising GOP star, Flake abandoned his bid for a second term largely because his support with Republicans plummeted after numerous clashes with President Donald Trump, often over the outspoken president's rhetorical style.

Flake, who turns 56 on Monday, published his final guest column as senator in The Arizona Republic on Saturday — two weeks after he delivered his farewell address in the Senate on Dec. 13. Flake is set to leave office after a single Senate term on Thursday, Jan. 3, when his successor, Democratic Sen.-elect Kyrsten Sinema, is sworn-in.

In the column, he expressed hope that Republicans can find common ground once again.

But Flake also included thinly veiled digs at Trump. He wrote that Americans have become too accustomed to the crude and course dialogues exchanged by political leaders — dialogue that wouldn't even be tolerated if it came from political opponents.

"Such theatrics may excite our political base, but that base becomes smaller with every new slight, with every new insult," Flake warned.

The Republican Party has become dominated by fear and conspiracy theories that have done nothing but cause further division, he said: "Lately, the party that has long been my political home has become defined by its resentments rather than its ambitions, and dominated by its fears rather than its principles. We have given ourselves over to conspiracy theories and given in to the impulse to vilify and divide, rather than exalt and unite.

He called the direction " beyond reckless ... quite simply, a prescription for political extinction."

America's lack of tolerance for incivility, he said, was demonstrated during the 2018 midterm elections when Republican candidates across the nation lost to Democrats.

"For a party that believes in free markets, the market sent us a resounding message through the midterm elections: While there is an audience for unkindness, that audience isn’t large enough carry the Republican Party to victory in most congressional districts and all but a few states," he wrote.

"For this, someday we will all be grateful."

Flake opened his column on an optimistic note, thanking Arizona for supporting him for the past two decades.

"I appreciate the support, encouragement, advice and even the criticism," he wrote to Arizona voters. "The beauty of our system is that people reign supreme and democracy works best when it is a vigorous conversation."

He thanked his family for putting up with a life in politics. He also invoked his time with the late Sen. John McCain, the six-term veteran who died Aug. 25 at age 81 after a battle with brain cancer.

"To serve alongside Senator McCain was an honor I never thought I would have — and was everything I thought it would be," Flake wrote.

Flake ended his column with a hopeful tone, saying Americans want to see a return to common ground. It's those hopeful and inspiring messages once embodied by the Republican Party that originally attracted Flake in the first place.

"Through all, and sometimes in spite of ourselves, we continue to move ever forward," Flake wrote.

He ended by remarking that he would get to witness Arizona swearing in two female senators for the first time in the state's history.

"Ever forward. For such progress, we can all be grateful," he wrote.

Follow Bree Burkitt on Twitter: @breeburkitt