Congress passed a temporary spending bill in the nick of time Saturday, averting a federal government shutdown for the next 45 days. But to secure support from the Republican conference, House Speaker Kevin McCarthy was forced to eliminate a provision that would have designated billions of dollars in additional military and other aid for Ukraine. Even with that concession, nearly half of House Republicans voted against the bill.

Backing for Ukraine’s resistance to the Russian invasion remains strong in Congress as a whole, and McCarthy is expected soon to file separate legislation to provide more aid for Kyiv. But the isolationist caucus in the GOP is clearly growing. Last Thursday the House voted to approve $300 million in Ukraine aid, but more House Republicans voted no than voted yes. (McCarthy was ousted as speaker on Tuesday.)

Increasingly, an “America First” mindset dominates the Republican base. In an August CNN poll, 71 percent of Republicans surveyed opposed further assistance for Ukraine. To some commentators, the mounting GOP opposition to any involvement in the Ukraine war is simply a return to the norm. “A streak of belligerent isolationism has always been present within the party,” Peter Juul wrote for The Liberal Patriot, a center-left newsletter. Jordan Tama, a professor at American University, observes in Foreign Policy that “an isolationist streak has existed within the Republican Party for more than a century, competing with a long-standing internationalist impulse.”

It is certainly true that Republicans have gone through phases of wanting America to pull up the gangplank and shirk the responsibilities that come with being an economic and military superpower. But so have Democrats. Both parties have proven to be highly susceptible to the isolationist virus. They are likeliest to succumb to it when the other party controls the White House — and to rediscover the importance of active engagement on the global stage when the president is from their side of the aisle.

Again and again, this has been the pattern.

After Iraq invaded Kuwait in 1990, President George H. W. Bush declared “This will not stand” and assembled a 42-nation coalition to roll back Saddam Hussein’s flagrantly illegal conquest. But when the time came for Congress to vote on authorizing the use of force, Democrats (future president Joe Biden among them) voted overwhelmingly against it.

Democrats had been more supportive two years earlier, when Bush sent troops to Panama to depose strongman Manuel Noriega after the killing of an unarmed US Marine. Yet even then, as The New York Times reported, “support from the Democrats was thin, and in some quarters nonexistent.” Bush’s decision was lambasted by senior members of the Democratic caucus, such as Representative Don Edwards of California, who called it “a trigger-happy” example of “our mindless, 100-year abuse of small Central American nations.”

While Republicans had firmly endorsed US action overseas when it was ordered by Bush 41, they were far less supportive a few years later when a Democratic president, Bill Clinton, deployed troops to the Balkans as part of a NATO effort to halt the Serbian slaughter in Bosnia. Traditional GOP internationalists like Senators John McCain of Arizona and Bob Dole of Kansas upheld Clinton’s policy, but hard-core Republican nationalists led by Patrick Buchanan insisted that America should wash its hands of Kosovo and leave the problem to the Europeans. “It is not our war,” Buchanan fumed. “This is an ugly civil war inside Yugoslavia. We have no vital interests there.” In May 2000, the Senate narrowly defeated legislation that would have forced Clinton to recall all US personnel from Kosovo — but a lopsided majority of Republicans supported such a withdrawal.

The 2011 military intervention in Libya that overthrew dictator Moammar Khadafy was ordered by Barack Obama, a Democrat — so it is not surprising that dozens of Republicans lined up to oppose it. A resolution by then-House Speaker John Boehner rebuking Clinton for having “failed to provide Congress with a compelling rationale … for current activities regarding Libya” was backed by virtually the entire GOP conference.

Conversely, when President Ronald Reagan ordered an invasion of Grenada following a Marxist coup d'état in 1983, it was Democrats — including six candidates for the 1984 Democratic presidential nomination — who deplored it. Tip O'Neill, the Democratic speaker of the House, condemned Reagan’s actions as a “frightening” example of “gunboat diplomacy.” Seven Democratic congressmen went so far as to introduce a resolution to impeach Reagan.

Yes, there is a rising tide of isolationism among conservatives and Republicans today, just as there was among liberals and Democrats during the Vietnam War and its aftermath. But in the ongoing tension between being the “world’s policeman” and the old yearning to avoid “entangling alliances,” where the parties stand at any given time is generally a reflection of who controls foreign affairs.

US support for Ukraine is the right course of action. But because the president is a Democrat, many Republicans think otherwise. They doubtless imagine that their opposition is motivated by pure principle. But if Biden were a Republican, the GOP would be a stronghold of support for Ukraine, and more and more Democrats would be calling for the aid to stop.