PS should let PSD form a minority government, wait for them to come up with an austerity budget, and promptly refuse to vote it. Then new elections would be called, and PS would be well positioned to win them.
Then PS declares new borrowing, Portugal's interest rates rise and borrowing becomes too expensive.
PS asks merkel for money.
Merkel says no.
PS backtracks and readopt austerity measures OR New elections
They win
Merkel still says No
PS backtracks and adopts austerity measures.
In case you think borrowing rates won't rise. It did back in 2011.
http://cdn.tradingeconomics.com/charts/portugal-government-bond-yield.png?s=gspt10yr&v=201510032352h&d1=20050101&d2=20151231