WASHINGTON (CNN) Eight of the major Democratic presidential contenders spent more than half of their Facebook advertising dollars over a two-month period on ads that mentioned either "debate" or "stage" -- a stark sign of how much new donor requirements to join nationally televised debates are driving campaign spending.

The spending on those topics was led by Julian Castro, whose campaign pumped nearly $335,000 -- or 87% -- of all his Facebook advertising into ads that mentioned the debates, according to a CNN analysis of data compiled by Bully Pulpit Interactive, a digital communications agency aligned with Democrats.

In second place: Washington state Gov. Jay Inslee, who spent nearly $278,000 on ads that mentioned the debate, or 70% of his total Facebook ad spend during the period examined.

The analysis covered advertising between March 30 and May 26. The ads may have mentioned other topics, but all included either of those two words.

Last month, following a flurry of last-minute pleas, Castro's campaign hit the 65,000-donor threshold -- one of two ways Democrats can qualify to participate in the first two Democratic National Committee debates. Candidates also can qualify by registering at least 1% in three polls.

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