Bush was elected President twice without a single EV from Pennsyvania (although admittedly by the skin of his teeth in 2000)
I think a more likely problem is that if GOP legislatures in swing states dropped the "winner take all" system because they never win statewide in Presidential elections, Dem controlled legislatures would neutralize the effect by doing likewise.
The good news is my first glance at the map shows there's not many areas where they could do so in order to erase new electoral votes the GOP would get out of FL/PA/OH/MI/VA/WI. The Dems could probably pick up quite a few electoral votes out of right-of-center moderate southern states if they switched to the Maine/Nebraska system, but those states are currently controlled by GOP legislatures so they'd stick with winner-take-all. The Dems would have the most to gain from changing Texas, at least on paper, since they virtually zero chance of carrying it statewide in a presidential election right now, but due to sheer population and congressional districts they could gain quite a few EVs by allocating them based Congressional district winner. There are large swaths of Dem-controlled areas in Texas, like the panhandle by the Gulf of Mexico and most of the major cities. However, the state legislature would never play ball and switch from winner take all. Arizona, same problem for the Dems on a smaller scale. The only state where they could use the legislature to change the system and might benefit is West Virginia, but it only has five electoral votes so at most they'd gain 1 or 2 EVs by that method.
WV hasn’t had a CD vote for the Democrat since 1996 (back when the entire state was heavily Dem), and all three CDs voted overwhelmingly for Romney. Switching to the ME/NE method would do nothing for WV Demcorats.
The Dems will not be able to win the TX senate and house for the foreseeable future even if they somehow won the governorship. Same thing for GA and other Southern states where they could do damage if they switched to a ME/NE scheme.
The Dems now control CO and NV, which were traditionally Republican in presidential elections, but switching to ME/NE in those states would only help the GOP, since both states gave Obama fairly comfortable majorities and would favor the Dem presidential nominee in 2016 (although both should be heavily contested).
So there are no mirror images of PA, MI, OH, FL, VA or WI controlled by the Democrats where they could counteract.