From Salon, this column by Glenn Greenwald:

There are few things rarer than a major politician doing something that is genuinely courageous and principled, but Jim Webb's impassioned commitment to fundamental prison reform is exactly that. Webb's interest in the issue was prompted by his work as a journalist in 1984, when he wrote about an American citizen who was locked away in a Japanese prison for two years under extremely harsh conditions for nothing more than marijuana possession. After decades of mindless "tough-on-crime" hysteria, an increasingly irrational "drug war," and a sprawling, privatized prison state as brutal as it is counter-productive, America has easily surpassed Japan -- and virtually every other country in the world -- to become what Brown University Professor Glenn Loury recently described as a "a nation of jailers" whose "prison system has grown into a leviathan unmatched in human history." [...] And, most important of all, Webb is addressing head-on one of the principal causes of our insane imprisonment fixation: our aberrational insistence on criminalizing and imprisoning non-violent drug offenders (when we're not doing worse to them). That is an issue most politicians are petrified to get anywhere near, as evidenced just this week by Barack Obama's adolescent, condescending snickering when asked about marijuana legalization, in response to which Obama gave a dismissive answer that Andrew Sullivan accurately deemed "pathetic."

Greenwald quotes at length from Webb's Senate speech (link provided). Here's the first quote:

Let's start with a premise that I don't think a lot of Americans are aware of. We have 5% of the world's population; we have 25% of the world's known prison population. We have an incarceration rate in the United States, the world's greatest democracy, that is five times as high as the average incarceration rate of the rest of the world. There are only two possibilities here: either we have the most evil people on earth living in the United States; or we are doing something dramatically wrong in terms of how we approach the issue of criminal justice.

Then, this on the Public Choice aspects:

It's hard to overstate how politically thankless, and risky, is Webb's pursuit of this issue -- both in general and particularly for Webb. Though there has been some evolution of public opinion on some drug policy issues, there is virtually no meaningful organized constituency for prison reform. ... Moreover, the privatized Prison State is a booming and highly profitable industry, with an army of lobbyists, donations, and other well-funded weapons for targeting candidates who threaten its interests.