Mr. Reid’s tendency to speak without inhibition or filter has created no shortage of complications and may have so alienated Republicans that they see no incentive to work with him. On Wednesday, Republican press offices, including Mr. Boehner’s, and Tea Party groups circulated remarks from Mr. Reid in which he appeared to be dismissive of cancer-stricken children. (In fact, he was ineloquently making a point about the need to fund the entire government, not just parts that Republicans have selected for special appropriations bills as a way to ameliorate the effects of the shutdown.)

Mr. Reid’s strategy to break Republicans depends on keeping his caucus unified, which is no small feat in a party as Balkanized as the Democrats can be. His colleagues said he understood all along that the only way Democrats could come out on top in a spectacle as politically harmful as a government shutdown was if they held together. With negotiations continuing, Mr. Reid declined to be interviewed.

So far, he is the only leader in Congress not to lose any of his members as the pressure rises. Mr. McConnell and Mr. Boehner, of Ohio, are both facing resistance from Republicans who represent states that have a mix of conservative and liberal voters. Representative Nancy Pelosi of California, the Democratic leader in the House, has also lost some members of her conference who hail from Republican-leaning districts and are reluctant to appear as if they are siding with President Obama.

“From the first time we talked about this, he said, ‘We are not giving in,’ ” said Senator Charles E. Schumer, Democrat of New York, who is one of Mr. Reid’s top lieutenants. “He had an instinctive understanding that this would work as long as Democrats didn’t fall for the bait. We haven’t, and we won’t.”

But Mr. Reid, an amateur boxer in his youth, is more than a smart strategic thinker, Mr. Schumer added: “Harry is a tough guy, and if you cross him he won’t forget it.”