Yesterday, in advance of President Donald Trump's trip to Canada for what promises to be one the most diplomatically testy G7 summits in modern history, his biggest Republican critic in the Senate, Arizona's Jeff Flake, went after the president on the topic burning the ears of America's biggest allies: trade.

I plan to speak on the Senate floor tomorrow morning to discuss the administration's protectionist policies and the importance of America's leadership in the world. Here is some of what I intend to say: pic.twitter.com/ox8HX75lsY — Jeff Flake (@JeffFlake) June 6, 2018

Flake yesterday also joined a bipartisan group of senators to introduce a bill requiring congressional approval of any presidential tariff imposed in the name of national security, a flimsy justification Trump has been using to absurd lengths. From the senator's press release:

The staggering negative impact of the administration's proposed tariffs is already being felt by workers and businesses across the country. Congress ought to assert leadership in this situation and take away the matches the president seems intent on using to ignite a dangerous trade war. I encourage my colleagues to promptly pass this legislation and push back against ill-conceived protectionist measures.

Before Flake could deliver today's lecture, Trump took to Twitter for some taunting:

How could Jeff Flake, who is setting record low polling numbers in Arizona and was therefore humiliatingly forced out of his own Senate seat without even a fight (and who doesn't have a clue), think about running for office, even a lower one, again? Let's face it, he's a Flake! — Donald J. Trump (@realDonaldTrump) June 7, 2018

The president is right about both the spelling and capitalization of the senator's last name, and I can testify after attending a fundraiser for Flake six days before he announced he wouldn't be running for re-election that there are some people who question the senator's reliability vis-à-vis future commitments. What's more, Trump is right about Flake's polls being very low—his approval rating was at 18 percent when he peaced out last October, although it has climbed back up to 32 percent since then. (Flake's approval/disapproval split, negative 18, was tied for worst in the first quarter of this year with Mitch McConnell's.)

It is also true that the presidential pacesetter for record low polling numbers in a first term remains Donald J. Trump, even after some modest recent gains. Gallup has Trump these days at 41 percent, close to but still lower than Jimmy Carter in early June 1978 (44 percent), Ronald Reagan in 1982 (45 percent), Bill Clinton in 1994 (46 percent), and Barack Obama in 2010 (47 percent). For the vast majority of his presidency, Trump's approval rating, when charted against those of his comparable predecessors, has been the floor.

What's more, as Gallup recently noted, "The percentage of Americans who strongly disapprove of the job Trump is doing is one of the highest for any president in the history of the Gallup 'strongly' question, which has been asked 82 times at irregular intervals." Trump registered a 41 percent "strongly disapprove" rating both times Gallup asked the question (in February 2017 and May 2018), while the share of respondents who strongly approved of his job performance dropped slightly, from 27 percent to 26 percent. According to Gallup, "only two presidents have had higher strong disapproval ratings": George W. Bush and Richard Nixon, both during their second terms.

Trump's polling softness may help explain why he's choosing to tweak Flake, rather than merely scraping him off the #MAGA windshield. The senator, who tried to rally Republicans against Trump long after the latter had secured the GOP nomination in 2016 and then targeted Trumpism in his 2017 bestseller Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle, is making noises about challenging Trump in the 2020 Republican presidential primary.

"It's not in my plans," he told Chuck Todd on Meet the Press on May 27. "But I've not ruled anything out. I do hope that somebody runs on the Republican side other than the president—if nothing else, simply to remind Republicans what conservatism is. And what Republicans have traditionally stood for."

If the national state of affairs remains largely as is between now and then, I would expect Flake—or (God help us) John Kasich, or (Lord have mercy on our miserable souls) Bill Kristol—to get creamed in a GOP primary. But one persistent feature of modern politics is that nothing stays stable for long. To state the obvious, Donald Trump wouldn't be president otherwise. So far 38 percent of Republicans tell pollsters that they want Trump to face a challenger in the 2020 primaries. As FiveThirtyEight pointed out in a useful explainer, that's pretty similar to Barack Obama's numbers among Democrats in 2010.

Still, we don't know what (if anything) the Robert Mueller investigation will turn up, how Congress will respond, how the composition of Congress will change this fall, or how the mercurial president will react to it all, let alone how the political context would change if, say, the economy turned sour or some unforeseen calamity occurred on the world stage. Maybe Mark Cuban throws his billions into the ring, maybe Justin Amash decides to go full Libertarian, maybe the L.P. finally vaults itself permanently into the double digits, maybe the Bernie-Hillary war becomes a full-fledged fracture on the left. America is just too weird right now to predict.

In the meantime, I'm glad there's a person on Team R objecting loudly to the party's recent conversion to mercantilism. If major-party ideologies are up for grabs right now, I want at least some people inside the tents fighting for economic sanity.

Speaking of which, here's Flake's speech today, in which he explains that "a trade war only guarantees that there will be losers" and complains that "this is not grown-up leadership":