Chesnot/Getty

Democratic lawmakers sent a letter to executives at Mastercard (MA - Get Report) , Visa (V - Get Report)  and privately-held Stripe, urging the companies to seriously reconsider their involvement with Facebook's (FB - Get Report) yet-to-be-launched cryptocurrency, Libra. 

Visa and Mastercard are part of the 27-company coalition known as the Libra Association whose stated goal is to develop a robust ecosystem of financial products that will allow people to use Libra everyday, around the world.

"We are concerned because key questions remain unanswered about the risks the project poses to consumers, regulated financial institutions, and the global financial system," Senators Sherrod Brown (D-OH) and Briat Schatz (D-HI) said in a letter. "Facebook has not yet demonstrated to Congress, financial regulators -- and perhaps not even to your companies -- that it is taking these risks seriously."

The letter was sent just two weeks ahead of Facebook CEO Mark Zuckerberg's visit to Capitol Hill to testify before the House Financial Services Committee on October 23.

The hearing, which is scheduled for 10 AM ET, is entitled "An Examination of Facebook and Its Impact on the Financial Services and Housing Sectors."

Payments provider PayPal (PYPL - Get Report) withdrew from Libra earlier this week. 

Facebook shares closed Wednesday's session up 1.2% to $179.85.

Facebook and Mastercard are key holdings in Jim Cramer's Action Alerts PLUS charitable trust.