The rhetoric about establishing Islamic government is a bit misleading.
There is no mandatory ruling system in Islam. There are mandatory rules and any system which effectively implements those rules is, by definition, Islamic.
We would do better to focus on Islamic policies; policies which are both derived from the shari’ah and that also promote conditions in the society which actualize Islamic values. The form the government takes is incidental; what matters is whether or not its policies adhere to Islam and lead to Islamic results.