Massachusetts governor Deval Patrick wants to be able to appoint someone to fill the Senate seat for the five month interim prior to the special election. A law needs to be changed. What's interesting is that the democrats changed the succession law in the first place:
The succession law was changed in 2004, when Sen. John Kerry, D-Mass., became his party's presidential nominee and Republican Mitt Romney was the state's governor. Democratic lawmakers changed the law to block Romney from installing a Republican to serve until the next general election. MSNBC