on a razor's edge
With the possibility of failing or changing drastically very suddenly or at any given moment. The stability of the market is on a razor's edge, as banks struggle to deal with a huge number of failing mortgages and bad debts. The policy norms for the country tend to be on a razor's edge, as the majority in congress constantly shifts between the two major political parties.
on a razor's edge
If a situation is on a razor's edge it could change or be decided at any time, often in a way that is dangerous. I knew my life at that moment hung on a razor's edge. In this election Ohio is balanced on a razor's edge between Republicans and Democrats.