Today, a federal court rightly struck down as unconstitutional Obamacare’s individual mandate. But this decision is only half right. It also shows that if Congress won’t repeal this law entirely, then tinkering with it might doom our chances in court of having this whole monstrosity thrown out.

Judge Henry Hudson of the U.S. District Court for the Eastern District of Virginia held that Obamacare’s individual mandate—requiring most Americans must buy health insurance—is unconstitutional. Although states might be able to require people to buy health insurance (like they do car insurance), the federal government cannot because it’s not authorized by any provision in the U.S. Constitution.

After correctly striking down the mandate, Judge Hudson then went in the wrong direction. Virginia Attorney General Ken Cuccinelli argued that the individual mandate cannot be severed from the rest of Obamacare’s 2,700 pages. As such, striking down the mandate means you have to strike down the whole law. Judge Hudson declined to take that step.

Cuccinelli’s right, and Judge Hudson—who’s a very good judge—got this one wrong. Hudson held that the record doesn’t make clear whether Congress intended the law to survive without the mandate, and that without such a record he should only strike down the part that’s clearly unconstitutional.

That’s not what Supreme Court precedent requires. As I explained in detail in a brief I filed on behalf of the Family Research Council in the multistate challenge to Obamacare in Florida, if a provision in a law is unconstitutional, a court must ask whether the statute can function in the manner Congress intended without the invalid part. If not, then the court must strike down the whole law.

There are two key points on this question of severability when it comes to Obamacare.

First, the law does not contain a severability clause, in which Congress announces that if part of the law is found invalid, the remaining provisions continue to carry the force of law. Courts treat a severability clause as strong evidence that Congress intended the rest of a law to survive without the unconstitutional section.

Even without a severability clause, a court still presumes an unconstitutional provision can be severed. It just doesn’t take as much to make the case that Congress would rather have no law at all.

That’s where the second point becomes critical. In Section 1501 of the Patient Protection and Affordable Care Act (Obamacare’s official name), Congress inserted a finding that the individual mandate, “is essential to creating effective health insurance markets.” Then, in their briefs and court arguments, the Justice Department admitted that the individual mandate is absolutely necessary for Obamacare to function as Congress intended.

The Virginia district court did not comment on this congressional finding and these government admissions. Taken in this context, the mandate is so intertwined with various parts of the law that a court needs to strike down many of Obamacare’s 450 sections.

Judge Hudson noted that he would have no way of knowing which provisions of Obamacare Congress intended to save without the mandate, also commenting that a number of provisions surely cannot be severed from the mandate. However, the correct course in that event is to strike down the entire law, allowing Congress to take the issue up all over again.

That raises a serious cautionary flag to Congress. The new Congress should do everything possible to repeal Obamacare entirely. However, if those efforts fail—as they likely will given that President Obama will veto any flat-out repeal, meaning a repeal couldn’t succeed before the 2012 elections—Republicans must not allow partial repeals to doom the court challenges to Obamacare.

Some moderate Democrats support a bill that would repeal the individual mandate. If that were to pass, then all of the major Obamacare lawsuits would become moot (since all of them challenge the mandate), and would be thrown out of court. Should that happen, then the rest of Obamacare would survive until 2013—or permanently.

Repeal Obamacare entirely. If Congress can’t, then it should be very careful, allowing the lawyers on these cases every opportunity to convince the Supreme Court that the individual mandate is unconstitutional, and cannot be severed from the remainder of the law.

If the Court holds the mandate unconstitutional and that it cannot be severed, then the whole law goes down, and we’ll kill this leviathan once and for all.

Ken Klukowski is a research fellow at Liberty University, and files briefs as special counsel on behalf of the Family Research Council in various Obamacare lawsuits.