If you think of most competing issues they are immediate and obvious. The economy - you see people without jobs, people losing homes, poverty. No one has to educate you on the problem existing. The wars - people are dying, there is hate and anger and it costs a fortune. Healthcare - the problem was very easy to define, the argument was on the solutions.
Immigration - is a bit more complicated and here it is seen as two very different problems - and both the left and right are fractured. On the right, the business side know the status quo provides cheap, exploitable label, but the RW side sees it as you said it is in TN. On the left, the labor unions may be closer to the "TN RW" than to the liberal social justice, effort to provide a path to citizenship. It is that complexity may end of being why this does not pass. In a bad economy, the labor part of the left might be even more reluctant to go along than in 2007. These factions are why there has always been more consensus that there is a problem, than there ever has been on any "solution". This comes from "immigration" as a problem meaning different things to different people.
Here is an interesting collection of polls -
http://pollingreport.com/enviro.htm Scanning through them, it is clear that one thing going against Kerry is that is that all the polls that try to balance the economy and the environment show that the willingness to sacrifice anything in the economy for the environment is at a low over the roughly 2 decade history. This is why Kerry speaking of this stimulating the economy - as he has since at least 2003 is a paradigm breaker. (consider that poll has been done for 20 years implicitly assuming there was a necessary tradeoff) This actually is where in 2004 he differed from Gore, who spoke of it more like a hard pill we had to swallow. You may have noticed that every 2008 candidate copied Kerry's approach on that. That and is indefatigable effort to convince business people of that possibility is why this effort is closer than it ever was.
The problem is that when you look out your window, everything looks just the same. For most of us, there is nothing visibly ominous. Unlike any of the other problems, including other environmental problems, it is all looking and believing data. That's very hard for most people. 1970 was easier - the Cuyohoga River was on fire! Pretty dramatic. The real problem is that even though some very major impacts are thought to be caused by global warming, they are not where most people are.
This really has to be legislation that is driven by leaders who are statesman seeing the need to do the right thing - and knowing that they are very unlikely to get applause for it. (Gore's Nobel was unexpected and I would bet that it was also intended as a poke in the face for GWB - as likely was Obama's before he did much and the very belated Carter one. )