This paper documents changes in investors' demand for sovereign debt during a debt crisis. Using a dataset containing individual bids on Portuguese debt auctions, I document that bid functions become more inelastic during the crisis. That is, investors require bigger drops in price to buy additional units of debt, increasing the government’s marginal cost of issuing debt. Authors then decompose these changes in demand into two components: a fundamental component, due to changes in the valuation of the security, and a strategic component, that arises from investors' market power. They find that, although the role of market power is negligible in normal times, it gets more pronounced leading up to and during the crisis. The government is not able to extract the full surplus from strategic investors, and, as a result, the auction mechanism loses efficiency during that period [...]