You wake up as Obama in 2014, the day before Crimea is invaded by Russia. What do you do differently, keeping in mind any possible Republican interference.
That's too late in the game for Crimea. Obama by this point has denied supplying weapons to the Ukrainians even after the Maidan and has been steadily eroding deterrent capacity. You would need time to deploy forward air assets to eastern Europe and fly joint air patrols with the Ukrainian people (preferably via NATO because the catalyst for the Maidan was the EU Accession Agreement, so increasing European presence will be more palatable to the Ukrainian citizenry). That takes time that Crimea doesn't have. So honestly, not much would change, because the invasion of Crimea would happen too quickly to prevent it from falling.
However, the undeclared Donbas War is still very much on the table. I would put the Budapest Treaty to the UN Security Council as a clear treaty violation. When Russia inevitably vetoes it, I'd call up Putin and ask him about the deniable forces in the Donbas. When he claims that they aren't Russian forces (which is something he did in real life), I'd say "Okay, then you won't mind if I eliminate them, then" and hang up, discreetly leaking the audio of the call to the press. I would then declare that the US remains committed to the Treaty's integrity and start bombing the Russian-led separatist forces in the Donbas. Also the entire security braintrust that promoted Russian appeasement gets shitcanned.
To handle the Republicans, I'd lean on Lindsay Graham to get enough of the hawks in Congress to support my action (Mitch McConnell at this point steadily devoted to obstructionism). I'd drafting trade arrangements to tie the US to Ukraine commercially, with the promise of profits to win over GDP-oriented Republicans and revitalizing the defense industrial sector in key swing states like Pennsylvania and Ohio. When Trump inevitably makes noise, selectively release information about his ties to Russian oligarchs and other figures (it's not exactly a secret that Trump relied on Russian funding to bail him out of his frequent bankruptcies) to call into question his motives when he claims "forever war."
Congress has already ceded much of its authority to the President, so I'd make the most of it and decisively force Russia to back down. If they continue, I'd hit back with some heavy sanctions packages straight from the get-go, and I'd use my good relationships with the EU to start selling oil and LNG to the EU to help with fuel prices, though the infrastructure to support it isn't quite there in Europe yet.
Thanks for the question, Anon.
SomethingLikeALawyer, Hand of the King