75,000

Hard to imagine any other candidate capable of this.  From Politico:

Sen. Barack Obama has seen his share of large crowds over the last 15 months, but his campaign said they have not approached the numbers gathered along the waterfront here right now.

The campaign, citing figures from Duane Bray, battalion chief of Portland Fire & Rescue, estimated that 75,000 people are watching him speak.

The scene suggests this is not an exaggeration. The sea of heads stretches for half a mile along the grassy embankment, while others watch from kayaks and power boats bobbing on the Willamette River. More hug the rails of the steel bridge that stretches across the water and crowds are even watching from jetties on the opposite shore.