From CATO:
Of course, if states refuse to set up an exchange, Obamacare gives the federal government the authority to step in and operate an exchange itself in those states. But there is reason to doubt that the federal government has either the ability or the money to do so. Congress has not appropriated any funding for this purpose and seems unlikely to do so.
More important, as my colleague Michael Cannon has discovered, a little-discussed provision of Obamacare makes federal subsidies for insurance available only through those exchanges that the states set up themselves. So, while the federal government does have the power to create exchanges in states that refuse to do so, it cannot offer subsidies through those federally run exchanges.
Moreover, it is those subsidies that actually trigger the penalty under Obamacare for employers who fail to provide workers with insurance. Obamacare requires employers with 50 or more workers to provide health insurance or pay a tax, but only if at least one employee qualifies for subsidies under the exchange. Therefore, if subsidies can be provided only through a state-authorized exchange, a state could potentially block the employer mandate altogether, simply by refusing to establish an exchange.