Russia's deputy PM Dmitry Rogozin has voiced the threat that's been a glaringly obvous Russian move for a long time;
"After analyzing the sanctions against our space industry, I suggest to the USA to bring their astronauts to the International Space Station using a trampoline."
So, it looks as if Russia is seriously thinking of cutting off US access to ISS, an opportunity Russia has due to the US being dependent upon rides on Soyuz to get astronauts to and from ISS.
If we were smart, we'd be working quietly with SpaceX and others to do several things. One is to prep a contingency cargo Dragon for crew transport (this could be done in a matter of weeks, easily.). I think it's a certainty that of all existing US space vehicles, cargo Dragon is the probably the closest thing we have to a crew-capable vehicle, so it's the obvious choice(1)*.
Another thing we should be doing is to examine and make contingency plans regarding options should the Russians decide to withdraw from the station (including their modules). This might not be as hard as it sounds; the Russian segment's critical functions are life support, control systems, and reboost, currently provided by the Zevezya module. But, it looks to me like most of those functions (at least to an adequate level) could be provided by a combination of the Interim Control Module http://en.wikipedia.org/wiki/Interim_Control_Module that NASA has in storage, plus the Zarya module that originally provided control module functions prior to Zevestya. Zarya is on the Russian segment, and was built by Russia, BUT, it's owned by NASA.
Getting the Interim Control Module to ISS won't be easy. It was designed for a shuttle cargo bay. I can't find mass figures for it, so I'm going to have to assume it's fairly massive as well as bulky. I can't find mass or dimensions, but as it was planned to use shuttle, we know the max it could be. The shuttle cargo bay was 15 feet by 59 feet. The F9 payload fairing is a bit wider (17ft), but it's shorter, at 43 feet. The biggest issue IMHO would be mass; the F9 1.1 can only do 29,000 pounds to LEO, so even less to the 51.6 ISS orbit, whereas the Shuttle could do 35,380 lb to ISS. So, if ICM is too heavy or bulky for F9, Delta Iv Heavy could handle it, both for mass and payload fairing dimensions.
I don't think Russia wants to commit on outright act of war (such as an armed takeover of ISS - something they have the capability to do now, due to their cosmonauts being armed) , but they very well might want to create a spectacular humiliation for the US.
Given the above, it might be enough to thwart a Russian attempt to doom the station (thus creating an enormous and expensive humiliation for the USA) via withdrawing their own modules. They might be quite willing to lose their own modules to accomplish this, so replacing their functions enough to allow ISS to avoid destruction (with either a skeleton crew or unmanned) might be possible. I'm not saying it is possible, just that it could be (and would behoove us to look, if we haven't already).
No matter whether or not ISS was worth building, it cost the US around 150 billion, and that's an investment we'd be insane not to take a few risks to preserve. There's also the aspect of allowing the Russians to humiliate us if they so choose, which they might.
The only question remaining is, is our leadership (in congress and the white house) insane/stupid enough not to understand this? Given their "safety first" mantra even under these circumstances, it pains me to say the answer is most likely yes.
*(1) there may be one other near-term option in addition to cargo Dragon. It's Dragon 2, also called Dragonrider, the manned version of Dragon. Per a tweet by Elon Musk, it's going to be unveiled in May (flight hardware, not a mockup or boilerplate model).