DES MOINES — With the final hours before the Iowa caucuses ticking away, Republicans are seizing on new and potentially damaging revelations about Hillary Clinton’s handling of sensitive information as evidence that she is unfit to be president.

The State Department revealed for the first time on Friday that information considered “top secret” was sent through Mrs. Clinton’s private email server while she was secretary of state. The agency also announced that it would not release 22 of her emails because they contained highly classified material.

The emails were not marked as classified when they passed through Mrs. Clinton’s server; in reviewing them more recently, officials determined their content should be kept from the public. But the revelations have provided new ammunition over the weekend for critics who say Mrs. Clinton has been dishonest and has put national security at risk.