Not leaving 2016 at the water's edge. (Petras Maluka/AFP via Getty Images)

Vice President Biden took a big jab at Donald Trump's foreign policy acumen while visiting Latvia on Tuesday, suggesting that Trump had no idea what he was talking about when he said the United States might not defend NATO allies.

"We have never reneged on any commitment we have made. Our sacred honor is at stake," Biden said in Riga, Latvia, according to The Post's Karen DeYoung. "The fact that you occasionally hear something from a presidential candidate ... it's nothing that should be taken seriously, because I don't think he understands what Article 5 is."

Article 5 is the portion of the North Atlantic Treaty Organization treaty that states "an armed attack against one [ally] or more of them in Europe or North America shall be considered an attack against them all." It's the part Trump has suggested he might simply ignore if NATO allies don't pay more toward the alliance. And that's something allies and American foreign policy experts are understandably concerned about.

But Biden didn't just offer assurances that the United States would abide by its NATO obligations; he also attacked Trump's knowledge of NATO — albeit without naming him — while on foreign soil. That's something that hasn't generally been done. American politics, since the middle of last century, were supposed to stop "at the water's edge," in the words of former senator Arthur Vandenburg (R-Mich.).

Biden, of course, is hardly the first to violate this unwritten rule — it has taken a beating during the 2016 campaign.

President Obama said during a May trip to Japan that Trump was rattling world leaders and attacked him for showing "either an ignorance of world affairs or a cavalier attitude" toward them.

Trump himself has taken part, too, attacking Obama and Hillary Clinton for opposing the United Kingdom's exit from the European Union — a.k.a. "Brexit" — during a June trip to Scotland.

"I was actually very surprised that President Obama would've come over here and he would've been so bold as to tell the people over here what to do," Trump said. "A lot of people don't like him. His recommendation perhaps caused it to fail."

It's no real surprise that the "at the water's edge" maxim is suffering so much. We live in an increasingly hyperpartisan political environment that makes no exceptions for foreign policy issues. And Trump's comments about pulling out of foreign agreements appear to be without precedent, coming from a major-party presidential nominee. Given the stakes involved and the doubts that Trump's comment might sow among U.S. allies, Obama and Biden are clearly making an exception to standard foreign policy procedure.

And Democratic voters will certainly give them a pass for it, believing nothing is sacred when it comes to stopping Trump and what they view as his dangerous foreign policy ideas.

But it is worth noting just how significant it is that this practice is being so roundly disregarded in this campaign. Even Obama and Biden criticized then-President George W. Bush for what they saw as a veiled attack on Obama, a Democratic presidential contender at the time, from the president during a 2008 visit to Israel.

"Some seem to believe that we should negotiate with the terrorists and radicals, as if some ingenious argument will persuade them they have been wrong all along. We have heard this foolish delusion before," he said, pointing to the Nazi invasion of Poland in 1939. He named no names. That didn't matter.

"The tradition has always been that when a U.S. president is overseas, partisan politics stops at the water's edge," then-Rep. and future Obama chief of staff Rahm Emanuel said. "President Bush has now taken that principle and turned it on its head: for this White House, partisan politics now begins at the water’s edge, no matter the seriousness and gravity of the occasion. Does the president have no shame?”

Biden called Bush's comments "bulls---."

"This is outrageous — for the president of the United States to go to a foreign country, to sit in the Knesset ... and make this kind of ridiculous statement," he said.

It was still enough of a rule last year that Wisconsin Gov. Scott Walker (R) cited it in declining to criticize Obama during a trip to London.

“Maybe it is a bit old-fashioned,” Walker said, adding: “I don’t think it’s wise to undermine the president of your own country” while overseas.

In 2012, Mitt Romney criticized Obama's policy on Russia while Romney's feet were firmly planted on American soil. But his comments drew a rebuke from then-House Speaker John Boehner (R-Ohio), because Obama was on foreign soil — in South Korea — at the time.

"While the president is overseas I think it's appropriate that people not be critical of him or our country," Boehner said.

Today, Boehner's and Walker's comments seem almost quaint. Perhaps the rule will again be observed once a presidential candidate isn't suggesting pulling out of long-standing foreign agreements.

But for now, "the water's edge" doesn't seem to be a meaningful line in American politics.