Congress voted in October to extend the debt ceiling through Feb. 7, and Lew said that the Treasury can keep spending for only a few weeks beyond that using extraordinary measures, according to the Huffington Post.
Republicans have already indicated that, despite their previous concession to pass a clean debt limit hike, they are looking to tack a key issue in exchange for another raise.
Senate Minority Leader Mitch McConnell (R-KY) on Tuesday said he doubted that Republicans would vote for a clean bill.
"I can't imagine it being done clean," he said.
Rep. Paul Ryan (R-WI), who helped broker a bipartisan budget deal that passed Wednesday, floated the idea that the GOP tie approval of the Keystone XL pipeline to a debt ceiling bill.
In his letter, Lew warned against such tactics.
"The creditworthiness of the United States is an essential underpinning of our strength as a nation; it is not a bargaining chip to be used for partisan political ends," he wrote. "Moreover, as you know, increasing the debt limit does not authorize new spending commitments. It simply allows the government to pay for expenditures Congress has already approved."