Bush has intentionally failed to properly enforce our immigration and border control laws for 6+ years. That is malfeasance of office, which is certainly impeachable.
Also because of this failure to enforce, he has allowed an unknown number of Islamic terrorists into the country. That is a violation of his Oath of Office to protect us against our enemies. That is unarguably impeachable.
Better include the senate and congress for about 30 years worth of neglect in this area.