A leading Republican senator maintains that President Obama is violating a campaign promise with his claim that he can bypass whistle-blower protections for executive branch officials who give certain information to Congress.

The lawmaker, Senator Charles E. Grassley, Republican of Iowa, sent a letter to Mr. Obama on Friday that condemned a signing statement the president attached to the $410 billion catchall spending bill he signed into law last week.

A signing statement, occasionally issued by presidents upon their signing a bill, is a document that instructs executive branch officials on how to carry out the new law. In this statement, Mr. Obama flagged a provision that protects officials who give information to Congress about their jobs or agencies. He said the statute could not limit his power to control the flow of certain information to lawmakers.

“I do not interpret this provision,” he wrote, “to detract from my authority to direct the heads of executive departments to supervise, control and correct employees’ communications with the Congress in cases where such communications would be unlawful or would reveal information that is properly privileged or otherwise confidential.”

In his letter, Mr. Grassley called Mr. Obama’s statement “overly broad” and said it would “undoubtedly chill whistle-blowers who might otherwise come forward to report waste, fraud or abuse to Congress.” He asked Mr. Obama to enforce and obey the statute fully.

“This is a shocking statement that acknowledges that you would be willing to give an order preventing employee whistle-blowers from making disclosures to Congress,” he wrote. “I do not see how this statement can be reconciled with your campaign promise to protect whistle-blowers. In fact, it is even more egregious than simply breaking a promise, because it actually restricts current and previously existing whistle-blower protections.”

Tom Devine, legal director of the Government Accountability Project, a group that promotes the defense of whistle-blowers, said Mr. Obama’s statement could mean very different things, depending on how broadly the administration defined the word “confidential.”

“The president has a responsibility to clear up the confusion,” Mr. Devine said.

The White House press office referred questions to an administration official, imposing the condition that he not be identified by name or title.

The official, a lawyer, said Mr. Obama was “committed to whistle-blower protections.” He declined to define every kind of instance in which the president’s power to keep a matter confidential would trump a whistle-blower protection statute, but he did say the administration had no intention of going further than did Presidents Bill Clinton and George W. Bush in signing statements concerning similar provisions.

“I don’t think President Obama’s signing statement injects a new level of uncertainty into the law,” he said.

During the presidential campaign, Mr. Obama criticized Mr. Bush’s frequent use of signing statements as an abuse of power and promised to exercise greater restraint when using them. His campaign said he would not issue “signing statements that undermine the legislative intent.”

In the letter Friday, Mr. Grassley told Mr. Obama he had violated that pledge, because “you have gutted the legislative intent of this provision.”

The administration official pointed to a memorandum Mr. Obama issued on March 9 laying out a signing statements policy. The document, which does not mention legislative intent, says he will employ only “legitimate” interpretations of statutes. Mr. Obama’s challenge in this case, the official said, is consistent with that principle.