Senators question ObamaCare hiring

But they questioned whether all of those jobs truly represented the "critical needs" for which direct-hire authority is usually reserved.

"Please explain why there was a 'critical need' for HHS to waive provisions of federal hiring law to hire 90 dental officers to implement the health care law," they wrote.

The letter to OPM also says HHS would have had to hire 10 people per day to fill all of the positions OPM designated as critical.

"The pace of hiring that would be required for HHS to maximize this authority and fill each of those 'critical needs' is incredible," the lawmakers wrote.

In addition to Coburn, the letter questioning HHS's hiring authority was signed by Sens. Richard BurrRichard BurrThe Trail 2016: Fight night Poll finds races for president, Senate tight in North Carolina Senate rivals gear up for debates MORE (R-N.C.), Michael Enzi (R-Wyo.), Charles GrassleyChuck GrassleyThe Trail 2016: Fight night Clinton, Trump tied in Iowa, Grassley leads in Senate race Senate rivals gear up for debates MORE (R-Iowa), Kelly AyotteKelly AyotteSenate rivals gear up for debates WATCH LIVE: Warren campaigns for Clinton in NH Green group endorses in key Senate races MORE (R-N.H.) and John ThuneJohn ThuneOvernight Tech: Tech pushes for debate spotlight | Disney may bid for Twitter | Dem seeks Yahoo probe Google backs Obama's internet transition plan Week ahead in tech: Key test for FCC's TV-box plan MORE (R-S.D.).