“Full disclosure and sunlight into all these expenditures is critical to getting to the root of this problem,” Mr. Rubio told the voter. He added, “As long as you know who’s behind the money and how much they’re giving and where they’re spending it, I think that’s the sunlight that we need.”

But as the fall campaign unfolds, voters in states like New Hampshire and Iowa have been left largely in the dark about who is putting the most money behind Mr. Rubio. Of all the television advertisements aired in support of the Florida senator so far this year — $5.5 million worth — none have been paid for by Mr. Rubio’s own campaign. Even the “super PAC” supporting him has not yet spent a dime on ads…

Unlike the super PACs, which are allowed to focus exclusively on elections, groups like the Conservative Solutions Project — known as a 501(c)(4) organizations, after the tax code provisions that govern them — are granted tax exemptions in exchange for devoting themselves to the promotion of “social welfare.” While social welfare groups may spend some of their time and money educating voters, they are not allowed to function as auxiliaries for a particular candidate. Asked how Conservative Solutions was dedicated to promoting public welfare, a spokesman insisted it was not aimed at boosting Mr. Rubio’s prospects…

But the group’s commercials all focus on Mr. Rubio. The senator’s picture is prominently featured on its website, and a video featuring Mr. Rubio speaking is the first thing shown to the site’s visitors. The group is now run by Pat Shortridge, an adviser on Mr. Rubio’s 2010 Senate campaign. J. Warren Tompkins, a South Carolina-based Republican consultant who was once a business partner with Mr. Rubio’s campaign manager, is on its board. Mr. Tompkins also oversees the super PAC backing Mr. Rubio — which is called Conservative Solutions PAC and which shares fund-raising consultants with the nonprofit.