Parking Options:
Best option is to park at the Tacoma Dome and ride the Link rail. Its free and will drop you off right by the rally. Runs every few minutes.
Tacoma Link Light Rail:
Free parking is available at the Tacoma Dome and the Light Link Rail has a stop right next to Tollefson Plaza at the Convention Center every 12 minutes.
http://www.soundtransit.org/Schedules/Tacoma-Light-Link-Rail.xml 
CLICK HERE FOR Map of Link and stops

Pay Parking Lots and Garages:
The areas outlined red on this map offer some parking options.  These are pay parking lots that offer hourly or multiple-hour parking for fees from $2 to $5.
Parking
Street Parking if available:
$0.75 per hour with a 2 hour time limit

Other helpful parking links:
http://www.tacoma.uw.edu/parking/campus-parking-lots-rates
http://www.tacomaartmuseum.org/page.aspx?hid=6813