"We believe the financial ties of the NFIB to corporate-funded political activist groups ... call into question the NFIB's role in speaking for small business interests," the co-chair wrote in a letter.
Within the next two weeks, the Supreme Court is expected to rule on whether the law is constitutional.
NFIB and other organizations that filed suit against the health care reform law argue that Congress exceeded its authority by mandating that people must buy health insurance by 2014 or face a penalty.
Spending reports filed with the Internal Revenue Service shows that conservative advocacy groups, such as Crossroads GPS, have donated millions to the NFIB.
"The Affordable Care Act or 'Obamacare,' is a major accomplishment that is already benefitting millions of Americans. The fact that NFIB has asked the highest court in the land to review the law -- and possibly overturn it -- means relevant information about participants ... should be disclosed," Ellison said. "We believe Americans should know who is funding a lawsuit that could take away their access to health care."
The NFIB has not directly responded to Ellison's request for information.
A spokeswoman for the NFIB said the organization has no plans to disclose its donor or member lists, adding that the allegations contained in Ellison's letter are without merit.
Here's a look at the letter:
and original article on Star Tribune