Republican Donald Trump loaned $1.8 million to his presidential campaign in the second quarter and raised another $100,000, fulfilling a promise he made in his campaign launch speech to self-finance his campaign.

Mr. Trump also spent a substantial amount of money: $1.4 million, or 74% of his total haul. That is the highest proportion of any 2016 candidate who has filed paperwork with the Federal Election Commission so far.

Mr....