Congressional Democrats sent a strongly worded letter to National Labor Relations Board member William Emanuel Tuesday demanding to know why he didn't recuse himself from a board case that reversed a major Obama administration-era precedent.

Emanuel had told lawmakers last week that he would clarify an earlier explanation about the issue. The NLRB Inspector General's office has reportedly started an inquiry into whether Emanuel violated ethics rules by not recusing himself.

The NLRB is the federal government's top labor law enforcement agency. Emanuel, a President Trump appointee and former private-sector lawyer, provided a key vote in a case late last year called Hy-Brand Industrial Contractors Ltd. that overturned the Obama-era board's controversial joint employer policy. Democrats argue Emanuel should have recused himself because his former law firm represented one of the clients in the case, called Browning-Ferris.

"By participating in the board’s action to bring [the Browning-Ferris case] back to the board, you are likely in violation of both federal regulations and the administration’s ethics pledge," wrote Sens. Patty Murray, D-Wash., Elizabeth Warren, D-Mass., Margaret Hassan, D-N.H., Reps. Bobby Scott, D-Va., Rep. Donald Norcross, D-N.J., and Delegate Gregorio Sablan, D-N. Mariana Islands.

Joint employer refers to when one business is so intertwined with a second one that it can be held legally responsible the second business's workplace policies. Until 2015, that required "direct control" of the policies. But after Browning-Ferris, the NLRB changed it to the much vaguer "indirect control," which vastly expanded legal liability for corporations, especially franchisers. The Hy-Brand case restored the standard to "direct control."

The Democrats initially wrote Emanuel in late January to ask whether he had participated in the Hy-Brand vote. In a written response Emanuel confirmed that he had and argued he was not obligated to recuse himself because the connection to his former law firm, Littler Mendelson, was too remote to merit that. However in a follow-up response last week, Emanuel said he would update that response.

In Tuesday's letter, the Democrats indicated that they were still waiting and added that they did not accept his earlier explanation. "You attempt to explain your actions by saying that you did not know that Littler represented a party in the BFI case. We would like to remind you that at the time of your confirmation in July 2017, you did in fact know that Littler represented a party in the BFI matter," the lawmakers noted, pointing to a response Emanuel made to a written question from Murray last year in which he was asked to list all cases decided by the board and that were on appeal in which Littler Mendelson represented a party.