If investors are looking for reassurance that the U.S. government will emerge from the upcoming debt ceiling negotiations unscathed, they might have been let down by Treasury Secretary Jack Lew Tuesday morning.

“I’m anxious. I’m cautious and anxious,” Lew told an audience at the Economic Club of Washington when asked to describe his feelings for buyers of Treasury bills about the looming showdown with Republicans and the possibility that it won't be resolved in time to prevent a government default.

Lew said tht he took comfort from knowing that congressional leaders are aware that default is unacceptable, but is made “nervous by the desire to drive this to the last minute when, the last minute is inherently unknowable and the risk of making a mistake could be catastrophic.”

Lew also cited the time the congressional process takes as another source of concern, saying that “if you have to march up the mountain three or four times before you get to the top” in negotiations, time can become an important constraint.

Government debt hit its ceiling in May, and Lew has been using extraordinary measures since then to keep the government functioning. He has said he will run out of options for meeting the government’s obligations in mid-October, and that the government risks the unthinkable possibility of a debt default at any time beyond then.

Despite his gloomy feelings, Lew does expect a deal to be struck before he runs out of headroom under the debt limit. “Congress will step up and deal with it,” Lew said, although he cautioned that he didn’t know what path Congress would take in coming to an agreement.

And the administration is still insisting it won’t be a part of negotiations.

“There are none,” Lew said of talks between the president and GOP leaders over avoiding a default. “There will be no negotiations over the debt limit.” He said that he has been talking to members of congressional committees to provide technical information about the timing of the Treasury’s maneuvers around the debt ceiling, but that those talks do not include negotiations.

Lew left the door open to a bargain over fiscal issues, though. He said that the president remains “anxious to find the sensible middle ground” on reducing the debt and noted that the president’s budget presented earlier this year reflects concessions he is willing to make to Republicans on spending and taxes.

Obamacare, however, is still off-limits as far as Lew is concerned. President Obama “will not accept measures that would tie a debt limit increase to defunding or delaying the Affordable Care Act,” Lew said in his speech.