"The privacy interests of donors is widely recognized and valued. Various public policy initiatives have rightly encouraged donations to social welfare organizations, and these efforts are threatened when private information about donors is not adequately protected," the letter states.

The new letter, from Sen. Orrin HatchOrrin Grant HatchProminent conservative passes on Utah Senate bid Republicans offer this impossible choice: Tax cuts or senior care Senate GOP running out of options to stop Moore MORE (R-Utah) and other GOP senators, says that, by requesting the confidential donor names, the agency has set in motion a process that will result in the names being available for public viewing at the IRS when the investigated groups are approved to become a 501(c)(4).

"The IRS appears to be circumventing the statutory privacy protections that Congress has long provided donors," exposing the donors to "possible harassment," the Republicans said."The public release of private donor information exposes citizens to possible harassment and intimidation by those who oppose the goals of the charitable organization."

Republicans and Democrats on Capitol Hill have been clashing over 501(c)(4) and other tax-exempt groups, some of which are spending millions of dollars this year on election advertising. 

The IRS currently says that the primary purpose of 501(c)(4) groups should not be political, which has led some legal analysts to assert that those organizations should use less than 50 percent of their budget on political causes.  

Watchdogs argue groups are misusing the 501(c)(4) tax-exempt status to keep their donors secret as they spend millions on election ads. 

The Republican senators requested that the IRS provide the statutory authority to require donor names, the frequency of requests for the information, and which officials were involved in requesting and approving the "questions requesting donor names," among other issues.

Along with Hatch, the Republican senators who signed the letter were Mitch McConnellAddison (Mitch) Mitchell McConnellAlabama election has GOP racing against the clock McConnell PAC demands Moore return its money Klobuchar taking over Franken's sexual assault bill MORE (Ky.), Mike EnziMichael (Mike) Bradley EnziGOP senators ask Trump for meeting on biofuels mandate Senate budget just the latest attack on seniors Week ahead: GOP's next steps on tax reform | Fed chief speculation heats up | Senate to vote on disaster relief MORE (Wyo.), Lamar AlexanderAndrew (Lamar) Lamar AlexanderGOP senator: ObamaCare fix could be in funding bill Collins: Pass bipartisan ObamaCare bills before mandate repeal Murkowski: ObamaCare fix not a precondition for tax vote MORE (Tenn.), John CornynJohn CornynGOP senator: ObamaCare fix could be in funding bill Senate GOP running out of options to stop Moore Texas Republicans slam White House over disaster relief request MORE (Texas), Kay Bailey Hutchison (Texas), Jon Kyl (Ariz.), Bob CorkerRobert (Bob) Phillips CorkerBannon: McConnell 'picking up his game' because of our 'insurgent movement' State Dept. spokeswoman acknowledges 'morale issue' The Hill's 12:30 Report MORE (Tenn.), Pat RobertsCharles (Pat) Patrick RobertsSenate ethics panel resumes Menendez probe after judge declares mistrial Senate passes resolution requiring mandatory sexual harassment training Overnight Energy: Perry takes heat for sexual assault comments | Clovis withdraws nomination for USDA post | Battle lines drawn on Arctic refuge drilling | Energy regulator back to full strength MORE (Kan.), John ThuneJohn Randolph ThuneSenate panel approves GOP tax plan Republicans see rising Dem odds in Alabama Overnight Health Care: Nearly 1.5M sign up for ObamaCare so far | Schumer says Dems won't back ObamaCare deal if it's tied to tax bill | House passes fix to measure letting Pentagon approve medical treatments MORE (S.D.) and Rand PaulRandal (Rand) Howard PaulCongress must end American support for Saudi war in Yemen Black men get longer prison sentences than white men for same crimes: study Sarah Palin on sexual harassment: 'People know I'm probably packing' so they 'don't mess with me' MORE (Ky.).

— Bernie Becker contributed to this report.