The bill requires individuals to buy insurance from private companies. If the government has the authority to tell you how to spend some of your money where is the limit? Can they tell you how to spend all of your money? Why not if the liberal claims that this is constitutional is correct? The commerce argument is very weak in my opinion because if you don't buy something you aren't participating in commerce at all...so how would the government then have claim to provenance under the commerce clause?
Now the other part of the argument then is based upon a bad ruling back in the 40's I believe it was where a farmer was prohibited from growing wheat beyond a government mandated quota even though he wasn't selling it and just using it on his own farm. SCOTUS ruled that by keeping it and not buying wheat on the market he was impacting the market price. But in this case the impact by one individual not buying health insurance has a non-existent effect on the insurance pool prices.
Ultimately, the liberals are twisting the meaning of the commerce clause in any case because the way they want it to be interpreted would give federal government complete control over every minute aspect of the lives of every American...congress would become a national dictator with an unrestrained interpretation of the clause...and there is absolutely no way that was the intent of the founding fathers.
The ruling in the 40s was fascist, and the idea that the federal government can prevent someone from growing food for their own family on their own property is one of the most egregious violations of liberty in recent memory.