While the president’s campaign had claimed that more than a million people had sought tickets for the rally, the 19,000-seat BOK Center was at least one-third empty during the rally. A second, outdoor venue was so sparsely attended that he and Vice President Mike Pence both canceled appearances there.

Tim Murtaugh, a spokesman for the Trump campaign, falsely blamed the small numbers on “radical protesters” and the news media who he said had frightened away supporters. But there were few protests in the area and no sizable effort to block entrances, and there was a strong security presence.

Mr. Trump was furious about the unused outdoor stage and the comparatively thin crowd in the stadium, according to two people familiar with his reaction. News broadcasts carried video of the partially empty stadium, and even the Drudge Report, a reliably conservative website, carried an all-caps headline that said “MAGA LESS MEGA” with a picture of rows and rows of empty blue seats.