I really do expect Democrats to maintain relevance by going in a militaristic and interventionist angle over the next few decades.)
Which actually would just be a return to the historical consensus. Conservatism being the more militaristic/interventionist force was a result of the neoconservative ideology of the Cold War, which carried into the War on Terror - and the relative isolationism/pacifism of the left results from the peculiarities of the Vietnam War. But historically, conservatives were the isolationists in American politics, while America's interventionist policies were largely shaped by progressives in both parties (both Roosevelts, Wilson, and Truman probably played the biggest role).
Huh? McKinley invented American imperialism.
He did, and by all accounts he was a conservative republican, so it doesn't always apply. But imperialism was inherited and supercharged by his progressive successor Teddy. Wilson, a progressive democrat, originally supported the Democratic Party's tradition of isolationism, but pulled off a rapid pivot to internationalism. The Republicans famously blocked the League of Nations and by the 1920s, the line between internationalist liberalism and isolationist conservatism was drawn more clearly, with Democrats favouring the former and Republicans favouring the latter (there were some differences of course, like southern Democrats largely being conservative on domestic issues but more in line with their party on the international stage).
So really, I guess 1900-1940 was the heyday of the era I'm describing where progressives became internationalist (one would argue out-and-out imperialist, especially in the case of TR), while conservatives slowly became skeptical of internationalism to the point of being total isolationists by the 1920s and 30s. But even though this era was started by the conservative McKinley, many foundational aspects of American internationalism like the Panama Canal and the League of Nations were advanced by progressives, while conservatives went on to take a more indifferent view of the rest of the world.
By the time of FDR, liberalism had basically won the debate as far as domestic issues went, but conservative isolationism won the foreign policy debate. Even after Hitler's invasion of Poland, public opinion in America remained largely isolationist (it has to be said that there were some high-profile Nazi sympathizers in the states, but they were far from the majority - most isolationist Americans had a "don't care, not our business" attitude). But as the war escalated, American public opinion started to shift. FDR started pivoting to greater activism on the international stage, and even Willkie did the same (much to the displeasure of many paleoconservative Republicans).
Then came December 7, 1941, a date which would live in infamy. In one fell swoop, the isolationist position became untenable, and conservatives slowly made peace with that reality. After the end of the war (and especially after the true extent of the barbarism of the Nazis and the Japanese came to light), a bipartisan consensus on America's role in the world started to emerge, and the Republicans who stuck to their paleoconservatism, like Taft, lost their influence as internationalist Republicans like Ike and Nixon emerged. The Democrats, of course, were full-blown internationalists at this point. Rebuilding Europe, stopping the rise of communism in the third world, and maintaining American hegemony became something both parties embraced.
As the Soviet Union continued to rise in a post-WWII world, the Republican Party (which at this point was the distinctly conservative party in national politics) became even more attracted to hawkishness. The Democrats were no less hawkish. But then came Vietnam, and many liberals (who at this point were mostly in the Democratic Party) started to oppose American imperialism. Humphrey had to delicately pivot away from LBJ's gung-ho interventionism, and McGovern was...well, McGovern. By the time of Carter, there was little doubt that even mainstream, moderate Democrats had become less interventionist than Republicans.
What we could now be beginning to see is a re-emergence of paleoconservatism in the Republican Party, in response to an increasingly internationalist, dare I say "globalist" Democratic Party. Bipartisan consensus is hard to maintain in a two-party system because opponents of one party's policies will gravitate to the other. Trump isn't an intellectually sophisticated ideologue, but he appeals to ideological paleo-cons in a way that Republicans haven't for generations. And neoconservatism has largely been discredited, rightly or wrongly, due to Dubya's foolish handling of Iraq.
