Theory v. Practice.

In theory, attorneys are not supposed to use opening statements to "argue" their cases.

In practice, they do so all the time.

Take today's high-profile Michael Jackson case, for example. According to thesmokinggun.com, Mr. Jackson's attorney started his statement with the following: "I'm here to tell you these charges are fictitious and bogus." He further argued: "These charges are fake, silly, ridiculous."
(HT: thesmokinggun.com.)

Sounds like argument to me, and a thesaurus exercise.