Democrats' billionaires tax explained

There is now legislative language behind the push to tax American billionaires on unrealized capital gains, as Sen. Ron Wyden last night released his 107-page plan.

Why it matters: This would be a sea change in U.S. tax policy, which has only applied to realized gains (otherwise known as income).

Wyden's proposal would apply to those with more than $1 billion in assets or more than $100 million in adjusted gross income over three consecutive years.

The biggest tax bills would come up front, charging a long-term cap gains rates on all...