After Sen. John McCain condemned “half-baked, spurious nationalism” as “unpatriotic,” President Trump publicly warned the Arizona senator to “be careful” because he would “fight back.”

In his acceptance speech for the National Constitution Center’s Liberty Medal, which was awarded to McCain for his “lifetime of sacrifice and service,” McCain did not mention Trump or his administration explicitly. But the implication seemed clear, and he emphasized that it was the country’s responsibility to be the “custodian” and “champion” of certain ideals at home and abroad, despite the costs of international leadership:

We have a moral obligation to continue in our just cause, and we would bring more than shame on ourselves if we don’t. We will not thrive in a world where our leadership and ideals are absent. We wouldn’t deserve to.

He did not hold back on his language:

To fear the world we have organized and led for three-quarters of a century, to abandon the ideals we have advanced around the globe, to refuse the obligations of international leadership and our duty to remain “the last best hope of earth” 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 that Americans consigned to the ash heap of history.

In response to the speech, Trump told a DC radio station, “People have to be careful because at some point I fight back.” He added, “And it won’t be pretty.”

Earlier on Tuesday, on Fox & Friends, Kellyanne Conway had argued that McCain’s comments were not directed at Trump. “Well, I just don’t see that in the president’s agenda or in his philosophy. His moorings are conservative, and he is governing as a center-right president who believes that we pay too much in taxes, that we’re over-regulated, that we have not taken terrorists seriously,” she said.

McCain has denounced Trump’s words and actions several times during both his campaign and his presidency, and Trump once infamously said McCain was “not a war hero” because he was captured. McCain, who continues to work a full schedule despite having been diagnosed with brain cancer in July, played a pivotal role in derailing Trump’s health care efforts, endearing him to many who see him as one of the few Republicans willing to stand up to Trump.

During his speech, McCain also talked about his military service, his time as a prisoner of war, and bipartisan cooperation. Joe Biden, who as the chairman of the center bestowed the medal on McCain, praised the Arizona senator’s decades of public service. “What you don’t really understand, in my humble opinion, is how much courage you give the rest of us in looking at you,” Biden said. Past recipients of the award have included the Dalai Lama, Malala Yousafzai, Hillary Clinton, and Muhammad Ali.

It’s not the only recent tiff with notable award winners: This year’s Nobel Peace Prize winner also called Trump a “moron.”