If you want to know why the Pennsylvania plan of rigging the Electoral College by having Republican majorities in the Keystone State change the way that electoral votes are allocated there is a bad idea in general, see Josh Putnam's excellent post on the topic. You remember -- this is the plan to neutralize the state in presidential elections by switching from winner-take-all apportionment of electoral votes to either a district plan or, now, splitting them by vote share. He's pretty mild on it, overall, concentrating on why the allocation is a poor one generally; he doesn't really go into why it's highly problematic for a party to constantly mess with the rules in such an ad hoc way in order to get the advantage.
But mostly, it's just not very likely to be a problem. The main thing here is that very few states which are reliably Democratic in presidential elections are going to have unified Republican government at the state level (and vice versa). Well, back up...first of all, the problem only arises in those states big enough so that how they split their votes will matter. Then after that, you need a state that's safely Democratic in presidential elections. If it isn't safe, then you run the risk of costing your own party!
And that's not all. After all, what matters here isn't how safe the state really is in presidential elections; it's what the incumbent Republican governor and legislature think. So for them to believe that neutralizing the state is good for a future Republican presidential candidate, they have to believe that their own elections were basically flukes. True, there are some who could acknowledge that, but I'd say it's more likely that they'll take their victories -- and remember, we're talking about unified control of state government, so it's a pretty wide win -- as signs that the state is trending to the GOP.
Everything to this point are reasons why there are only rare cases in which a state is going to find it in the national party's interest to pass this kind of plan.
The next step is that what's in the national party's (perceived) interest is unlikely to be in the interest of the state or the state party or the individual politicians. That's because each of them (the state, the state party, and the individual politicians) all have an interest in keeping their state as a major presidential battleground. The reason is the same in each case: resources flowing into the state are good, and national resources follow electoral incentives. It's possible that state politicians will nevertheless act out of national party interest, but it's not obvious they would do so at all.
And the last part? Generally, something like this doesn't just have to be favored by Republican politicians; it has to be a fairly high priority, or it won't get action. It's certainly something that will be perceived by everyone as a flat-out partisan move, and presumably Republicans who have won office in a Democratic-leaning state -- who, per above, believe that it's a Democratic-leaning state so they won't get too many chances -- won't want to pass too many flat-out partisan measures, and will have several items more important to them than a electoral college scheme that may never make any difference anyway.
Put it all together, and you're just not very likely to have this ever happen (and it's even less likely it would happen in more than one state, or stick around for long in that state). So: terrible idea, but not one to worry too much about.