WASHINGTON, United States (AFP) — US President Donald Trump reignited a long-standing feud with John McCain yesterday, warning “I fight back, and it won’t be pretty“, after the respected senator appeared to take aim at him in a speech.

The relationship between the pair — one a veteran lawmaker and war hero, the other a real estate tycoon and political neophyte before he assumed the presidency — has long been testy.

But the acrimony has risen to a new level with Trump’s latest threats, which come just over a week after a war of words with another senior legislator from his own party, Bob Corker.

Speaking on Monday night at an event in Philadelphia to honour his life’s achievements, McCain had railed against the “America First” approach espoused by the Trump Administration.

“To refuse the obligations of international leadership… for the sake of some half-baked, spurious nationalism cooked up by people who would rather find scapegoats than solve problems, is as unpatriotic as an attachment to any other tired dogma of the past,” said the 81-year-old, who was diagnosed with brain cancer in July.

Trump lashed out yesterday, telling conservative radio host Chris Plante: “People have to be careful because at some point I fight back.

“I’m being very nice. I’m being very, very nice. But at some point I fight back, and it won’t be pretty.”

McCain has proved a thorn in Trump’s side in recent months, twice helping to scupper Republican attempts to repeal a health care law that was a signature accomplishment of former President Barack Obama.

Asked by AFP what he made of the president’s latest missive, McCain shrugged the matter off.

“I do what I have to do and I work with him as much as I can, and I do what I need to do for the people of Arizona,” he said.

Regarding the president’s suggestion he would fight back, he responded simply: “I have faced some pretty tough adversaries in my life.”

The Trump-McCain feud has a long history.

Trump — who in addition to being a businessman was also a reality TV star winning the White House — questioned McCain’s military record in July 2015, earning widespread rebuke for saying “I like people who weren’t captured” in comments that threatened to derail his presidential ambitions just a month after announcing his bid.

Former Navy pilot McCain spent more than five years as a prisoner of war in North Vietnam where he was tortured, while Trump received student and medical deferments that prevented him from being enlisted in the conflict.

In August, McCain broke party ranks to call Trump “often poorly informed”, adding he “can be impulsive in his speech and conduct”.

Some political observers wondered whether his votes against Republican attempts to repeal Obamacare — one of Trump’s main campaign promises — were influenced by the desire for revenge.

McCain on Monday received the backing of another long-time Trump critic Mitt Romney, who was also the Republican candidate for president in 2012.

“Ran against him, sometimes disagree, but proud to be a friend of @SenJohnMcCain: hero, champion of character and last night, Lincolnesque,” said Romney.

The latest battle with a senior member of the Republican party comes after Trump engaged in a heated exchange with senior Senator Bob Corker, who accused the president of pushing the country to the brink of World War III.

Trump’s frequent sparring with congressional leaders is seen as perhaps damaging to his legislative agenda, with a potentially bruising fight for tax reforms that could divide his party on the horizon.

Republicans currently hold only a slim majority in the Senate, with 52 of the chamber’s 100 seats.