DES MOINES — Hillary Rodham Clinton used a speech here to tie herself closely to President Obama and attack Republicans on Saturday, avoiding criticism of her two rivals for the Democratic nomination after a string of recent successes that have lifted her candidacy.

In sharp contrast, Senator Bernie Sanders, after saying for months that he did not want to attack Mrs. Clinton, came at her hard on Saturday night, criticizing her as a shape-shifting politician and inconsistent progressive.

Speaking at a party fund-raising dinner held 100 days before votes are cast at the Iowa caucuses, a forum that has traditionally inspired Democratic presidential hopefuls to sharpen their attacks on their rivals, Mr. Sanders, and to a lesser degree former Gov. Martin O’Malley, did so once again.

In the wake of a strong debate performance, Vice President Joseph R. Biden Jr.’s decision not to run and polls showing her on the rise, Mrs. Clinton gave a familiar address that hewed closely to her usual stump speech.