Who? Reagan? Not really a fair comparison. The country's changed a lot since the 80s and so has what it means to be a conservative.
A lot of Reagan's success relates to when he ran. You might as well say that Democrats should copy FDR or LBJ, because they racked up the largest percentage of the vote.
If you're talking about conservative Senators or Congressmen who got a higher percentage of the vote than Romney, didn't that tend to average out across the country -- Romney running ahead in some places and behind in others?
But that still doesn't answer the question about how many stay-at-homes were ideological conservatives and how many just didn't want to vote for the clueless rich guy, or about how many people specifically refused to vote Romney and how many had already given up on the GOP before 2012.
I don’t think there is any statistical evidence that significant numbers of conservatives voters stayed home in 2012. Certainly not enough to have flipped any state.