Widening their efforts to go after Hillary Clinton, Republicans in the Congress have asked the Internal Revenue Service to review the tax-exempt status of the Clinton Foundation, arguing its failure to report foreign donations have raised questions about the group's work.

"As a result, given the substantial public interest involved, we feel a prompt review of the Foundation's tax-exempt status is appropriate to determine whether it is acting within the scope of its charitable mission," read a letter from a group led by Rep. Marsha Blackburn (R-TN).

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Credit: Jamie Dupree

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Credit: Jamie Dupree

"The allegations swirling around the Foundation are very serious and should be thoroughly vetted," Blackburn said.

The Blackburn letter, which was signed by 50 other Republican lawmakers, can be read here.

If Clinton herself had any worries about the Foundation's work, it didn't show in Iowa, where she took a few questions from reporters for the first time in almost a month.