In late September 2016, Cambridge and other data vendors were submitting bids to the Trump campaign. Then-candidate Trump’s campaign used Cambridge Analytica during the primaries and in the summer because it was never certain the Republican National Committee would be a willing, cooperative partner. Cambridge Analytica instead was a hedge against the RNC, in case it wouldn’t share its data.

The crucial decision was made in late September or early October when Mr. Trump’s son-in-law Jared Kushner and Brad Parscale, Mr. Trump’s digital guru on the 2016 campaign, decided to utilize just the RNC data for the general election and used nothing from that point from Cambridge Analytica or any other data vendor. The Trump campaign had tested the RNC data, and it proved to be vastly more accurate than Cambridge Analytica’s, and when it was clear the RNC would be a willing partner, Mr. Trump’s campaign was able to rely solely on the RNC.