On his blog, Dani Rodrik argues that
the fact that an international rule is negotiated and accepted by a democratically elected government does not inherently make that rule democratically legitimate.
Rodrik distinguishes two types of international commitments. On the one hand, there are commitments that help to overcome time-inconsistency problems.
[For example, the government] would like to commit to free trade or to fiscal balance, but realizes that over time it will give in to pressure and deviate from what is its optimal policy ex ante. So it chooses to tie its hands through external discipline. This way, when protectionists and big spenders show up at its door, the government says: “sorry, the WTO or the IMF will not let me do it.” Everyone is better off, save for the lobbyists and special interests. This is the good kind of delegation and external discipline.
On the other hand, there are commitments that mainly serve to tie the hands of current or future political opponents.
From an ex-ante welfare standpoint, this strategy has much less to recommend itself. The future government may have better or worse ideas about government policy, and it is not clear that restricting its policy space is a win-win outcome. This kind of external discipline has much less democratic legitimacy because, once again, it privileges one set of interests against others.
In an earlier contribution, I have argued that a key role of the European Union should be to play the former role.