How can you lose a referendum and then win the election?

Professor John Curtice writes at FiveThirtyEight on the SNP and the upcoming General Election vote in Scotland:

There are three main parts to the answer. The first is that May’s election is being held under very different rules from the referendum. Second, the question of what Scotland’s constitutional status should be now matters more to voters than it did in the past. And third, the SNP has come to be regarded by many voters as the party keenest on creating a more equal society.

At 45 percent, the average level of support for the SNP in current Scotland-wide polls matches the percentage that the “yes” side won in the referendum. Indeed, it also equals the 45 percent that the SNP won in the election to the devolved Scottish Parliament held in May 2011. The crucial difference is that whereas 45 percent is always insufficient to win a referendum, it can be enough to win a landslide in an election that is held under the first-past-the-post electoral system and in which a multitude of parties are in contention.

This is a good primer for anyone wondering how you can go from losing the referendum on Independence in September 2014, and then potentially hold the balance of power in Westminster in May 2015.