I would note that this is also nothing new. The first time Steve Rogers gave up being Captain America? Nixon Adminstration. (OK, technically, Ford Administration, but Watergate was the catalyst.) The second time? Reagan Administration. He died during the Bush Jr. Administration. None of these were accidents. Marvel's creators have never been shy about making their liberalism known, and have always shown it off more when Republicans were in power.
I'd also disagree that Marvel really is pushing a Democrat vs. Republican thing right now. Captain Samerica, sure. Spencer's definitely portraying conservatives as antagonists. But even then, not exclusively. He used Rage for a "violence is not the answer" message, and he had some really goddamn stupid "SJW" types as part of a "look the left can be stupid and extreme too!" thing.
But other than Captain Samerica? There's really not much in the way of "conservatives are bad" messages. Mighty Thor isn't anti-Republican, it's anti-Patriarchy. Power Man & Iron Fist doesn't deal much with politics, even when the CWII arc got deep into injustices in the justice system. Ms. Marvel is a distinctly message-heavy book, but those messages are hopeful and not specifically about any political orientation.
There's been a couple pot-shots at Trump, and I would expect to see a lot more of those going forward, because Trump is just an abominable person and has no business being anywhere near the White House. But hell, even those shots aren't partisan, they're not about Republicans, they're about how genuinely awful and incompetent Donald Trump is, and his being awful and incompetent and corrupt has nothing to do with him being a Republican.