The Washington battle over a pair of Labor Department appointments is becoming more tense as the Senate’s holiday recess approaches.

All 47 Senate Republicans sent a letter to President Obama on Monday requesting he withhold from appointing Deputy Labor Secretary Sharon Block and union lawyer Richard Griffin to the National Labor Relations Board (NLRB) during the mandatory adjournment between sessions of the 112th Congress.

In the letter, Republicans wrote that a decision to recess appoint Block and Griffin would “set a dangerous precedent that would most certainly be exploited in future cases to further marginalize the Senate’s role in confirming nominees and could needlessly provoke a constitutional conflict between the Senate and the White House.”

Since 2009, Obama has made 28 recess appointments, all to full-time positions. During his eight year tenure, Obama’s predecessor, Republican George W. Bush, made 171 recess appointments, of which 99 were to full-time positions.

Republicans have been critical of the NLRB, arguing it has been too easy on unions. The board currently has two vacancies to fill. Sen. Lindsey Graham (R-South Carolina) has vowed to block any appointments through the Senate confirmation process.

Sen. Orrin Hatch (R-Utah) led the effort against Obama, stating in the letter that “appointments to the NLRB have traditionally been made through prior agreement of both parties to ensure that any group of nominees placed on the board represents an appropriate political and philosophical balance.”

When the Senate is in recess, the President has the Constitutional authority to make a temporary appointment, called a recess appointment, to high-level policy-making positions in federal departments, agencies, boards, and commissions. A recess appointment expires at the end of the Senate’s next session or when an individual is nominated, confirmed, and permanently appointed to the position, whichever occurs first. Rejection by the Senate does not end the recess appointment.