Remember the context: Mankiw loved the Bush-era fiscal policies to create long-run structural budget deficits, and worked hard to implement them--the unfunded war and unfunded tax cut and unfunded entitlement policies that did so much to create our structural deficit. Mankiw did his best to join in the process of taking the work that we in the Clinton administration had done in the 1990s to restore America's fiscal balance--work that was very well done, very important, and work that we were and are very proud of--and casually smashing it on the floor.
But Republicans will inevitably see a balanced budget as an opportunity to give money to rich people (tax cuts and crony capitalism). The reward for liberals for this well done very important work? Tax cuts for rich people and unpaid for disastrous wars.
Liberals should spend their time in office figuring out how to implement a sticky liberal agenda, one which is hard to dislodge, not figuring out how to create a pot of money for Republicans to steal when it is their turn.