BERLIN — Chancellor Angela Merkel of Germany had hoped to focus her re-election campaign on domestic priorities like energy and health care, but revelations of far-reaching American surveillance programs and persistent questions about whether they may have violated the rights of German citizens continue to intrude.

For the second time in little more than a week, Ms. Merkel, 58, publicly addressed questions Friday about the extent to which the United States might have listened in on Germans’ telephone calls and monitored their Internet communications. “We are examining what happened, whether this is the tip of the iceberg, or less serious, or something else — what is true,” she said in response to reporters’ questions at her annual news conference before breaking for summer vacation.

Although Ms. Merkel, who is seeking a third term, and her center-right Christian Democratic Union maintain a comfortable lead in polls in advance of the presidential election in September, the scandal appears for the first time to be chipping away at Germans’ confidence in her leadership. More than two-thirds of Germans said they were dissatisfied with her government’s attempts to answer allegations that the United States used its massive surveillance program to spy on Germans, a survey by Infratest Dimap for the ARD public network showed.

“I am the head of government, and consequently, I have to make sure that here in our country German law has been upheld,” Ms. Merkel told reporters, making a fist to drive home her point. “In Germany, as in Europe, the right of the powerful does not override the power of the law.”