As Ted Cruz tells it, the story of how he financed his upstart campaign for the United States Senate four years ago is an endearing example of loyalty and shared sacrifice between a married couple.

“Sweetheart, I’d like us to liquidate our entire net worth, liquid net worth, and put it into the campaign,” he says he told his wife, Heidi, who readily agreed.

But the couple’s decision to pump more than $1 million into Mr. Cruz’s successful Tea Party-darling Senate bid in Texas was made easier by a large loan from Goldman Sachs, where Mrs. Cruz works. That loan was not disclosed in campaign finance reports.

Those reports show that in the critical weeks before the May 2012 Republican primary, Mr. Cruz — currently a leading contender for his party’s presidential nomination — put “personal funds” totaling $960,000 into his Senate campaign. Two months later, shortly before…