One theory is that an abundance of American Christians believe in dispensational premillennialism[0], which rebukes orthodox Judaism in the hopes of returning Christ. Many dispensationalists (including Issac Newton[1]) attempted to interpret the Bible as a function of divine will, and argued that the foundation of Israel is
directly imperative due to the prophecy of Daniel. This seems to go against the majority orthodox Judaism interpretation that the Third Temple (as well as the return of Christ) is willed by God alone and not predetermined conditions that men can control.
It's a bit out-there, but unfortunately I can't write-off DJT accepting it all at face-value. He's got conspicuous in-laws and an awfully weird track-record writing policy for the Levant. A religious conviction to defend Israel on behalf of his savior seems to slot rather neatly into his internal belief system.
[0] https://en.wikipedia.org/wiki/Premillennialism#Dispensationa...
[1] https://www.gutenberg.org/ebooks/16878