Big donors like David H. Koch and George Soros could owe taxes on their millions of dollars in contributions to nonprofit advocacy groups that are playing an increasing role in American politics.

Invoking a provision that had rarely, if ever, been enforced, the Internal Revenue Service said it had sent letters to five donors, who were not identified, informing them that their contributions may be subject to gift taxes depending on whether the donations exceeded limits under the tax laws.

These advocacy groups have been drawing more scrutiny, from President Obama as well as others, as they have proliferated and funneled vast sums of money in support of campaigns and causes, without having to publicly disclose their donors.

During the midterm cycle, for example, groups like Crossroads GPS, which has ties to the Republican strategist Karl Rove, and Americans for Prosperity, backed by Mr. Koch and his brother Charles, were heavily involved in politicking, spurring campaign finance watchdogs to complain that they were flouting election and nonprofit laws.