As explained in this New Republic essay by Si Lazarus, a new challenge to the Affordable Care Act is nearly as threatening to its viability as the now-rejected challenge to its constitutionality. Here's an excerpt:
After the Supreme Court upheld the Affordable Care Act last June, Senator Jim DeMint and Representative Michele Bachmann wrote Republican governors, urging them to refuse to establish ACA-prescribed “exchanges”—statewide health insurance markets—for small businesses and individuals not covered by employer-sponsored health plans. At the time, it seemed unlikely that many governors would follow this cut-off-your-nose-to-spite-your-face advice, since the ACA directs the Federal government to step in and run an exchange for states if they opt out. Hence, no real harm to the ACA, just to Republican governors and legislators, who would lose credit for a valuable constituent service. But Tea Partiers have kept up their resist-resist-resist drumbeat, and over half the states have stuck with the refusenik option. ... On May 2, the [resisters] ... filed in the District Court for the District of Columbia a complaint backed by the Competitive Enterprise Institute. [The plaintiff's lawyer Michael] Carvin’s legal argument, which originated with CATO Institute economist Michael Cannon and Western Reserve law professor Jonathan Adler, is that, due to a drafting glitch in the ACA, only state-run exchanges, not federal ones, can provide tax credits and subsidies to enable lower-income individuals to afford ACA-mandated health insurance. Threading the Carvin-Cannon-Adler argument through the intricacies of the ACA and the Internal Revenue Code could make even a seasoned tax lawyer’s head hurt. But the bottom-line is not hard to grasp: If the case makes its way to the Supreme Court, and all five members of the Supreme Court’s conservative bloc buy Carvin’s rationale, the result would, in rejectionist states, subvert the ACA’s central purpose and stiff the very population the law was enacted to benefit. The Obama administration has estimated that 80 percent of the millions of individuals expected to purchase insurance through exchanges will require some tax subsidies. Without those subsidies, in states with Federally facilitated exchanges, the individual mandate, constitutional though the Court has declared it, simply won’t work.
In sum, the effort to kill the ACA continues. The new court challenge is Halbig v. Sebelius, No. 13-63 (D.D.C.). Here's the Halbig complaint.