Nicola Sturgeon thinks the onus is on Ed Miliband to make a deal with her or else he will be responsible for letting David Cameron back into Downing Street.
The onus is in fact on the SNP. The SNP can either vote with Labour in the House of Commons or against Labour and with the Conservatives.
Ed Miliband will either be Prime Minister or leader of the opposition. Nicola Sturgeon can choose whether she supports Miliband or not, but I doubt her supporters in Scotland or the rest of the UK will forgive her if she fails to do so after all her anti-Tory rhetoric.