Donald Trump doubled his campaign expenses last month, yet was still spending at a far slower rate than Hillary Clinton.
New finance documents show the Republican presidential nominee's campaign spent about US$18.5m in July.
By comparison, the Democratic presidential nominee spent about US$38m.
The New York businessman gave his campaign another $2m in recent weeks. He had pledged to contribute as part of a fundraising pitch to small donors, telling them he would match their efforts up to that amount.
In all, his campaign brought in $37m for the month. Clinton's campaign raised $52m.
Mr Trump has continued to increase the size of his operation, putting $5m this week into his first batch of TV ads.
Mrs Clinton has been running general election ads for months.
Last month Mr Trump brought in some new consultants and firms, July reports to the Federal Election Commission show.
At the end of July, the campaign paid $100,000 to Cambridge Analytica, a deep-dive data firm that did business with Republican opponent Ted Cruz.
Hedge fund billionaire Robert Mercer, who contributed $2m to a pro-Trump super political action committee in July, is an investor in Cambridge.
The Trump filings also show some old ties.
Two weeks after campaign manager Corey Lewandowski was ousted, the Trump campaign sent his firm, Green Monster Consulting, another $20,000 cheque.
That is about the same amount it had paid him each month while he was running the campaign.
At the time of the latest payment, Mr Lewandowski was already on the payroll of CNN, where he is a political contributor.