That seemingly flat-footed reaction, several Democratic strategists said, illustrated how Mrs. Clinton’s decision to delay a formal start to her campaign had left her vulnerable. She has waited until recent weeks to begin hiring staff and creating the sort of sprawling infrastructure a presidential bid demands. She also only recently hired researchers to start reviewing her paper trail from the State Department, the Clinton Foundation and the past two years as a private citizen, to identify potential trouble spots that might need to be addressed later in the campaign.

The allied groups that are devoted to defending her, meanwhile, are mostly constrained to following her lead. Mrs. Clinton’s aides did not alert officials with the Democratic National Committee or outside liberal groups such as American Bridge that the Times article was coming, according to Democrats briefed on the matter, leaving her would-be defenders scrambling and hastily searching for similar transgressions by Republicans that could at least muddy the issue.

Late Wednesday, Mrs Clinton posted a message on Twitter saying that she had asked the State Department to make her emails public, but also said that they were still under review. By then it had been revealed that the email server she had used had been regiestered at her home address in Chappaqua, N.Y., and established by a longtime political aide.

The episode also crystallized the difficulty Democrats are experiencing at a time when Mr. Obama is still the head of the party, but Mrs. Clinton, an undeclared candidate with no campaign apparatus, is the one being scrutinized by the news media and attacked by Republicans.

“Our nominee is not going to be named Barack Obama, and we better get prepared for that and do it fast,” said Bill Carrick, a veteran Democratic strategist. Part of the problem, Mr. Carrick noted, is that Mr. Obama created a parallel political infrastructure outside the Democratic National Committee.

“That’s fine when you have a president in the White House to respond to things on daily basis,” he said. “But that’s not going to work in terms of the campaign coming up. And it has started — there’s no way of getting around that. The Republicans are out there trash-talking her.”

Edward G. Rendell, a former Pennsylvania governor and the chairman of the Democratic National Committee during the 2000 election, said Mrs. Clinton needed a more robust political organization to defend her.