Conservative strategist Stephen K. Bannon oversaw Cambridge Analytica’s early efforts to collect troves of Facebook data as part of an ambitious program to build detailed profiles of millions of American voters, a former employee of the data-science firm said Tuesday.

The 2014 effort was part of a high-tech form of voter persuasion touted by the company, which under Bannon identified and tested the power of anti-establishment messages that later would emerge as central themes in President Trump’s campaign speeches, according to Chris Wylie, who left the company at the end of that year.

Among the messages tested were “drain the swamp” and “deep state,” he said.