CHARLOTTE, N.C. — A torrential downpour that struck Charlotte on Saturday afternoon damaged the Mount Rushmore-style sand sculpture bust of President Obama — an ominous beginning to what many fear is a plagued convention.

Workers were trying Saturday afternoon to reform the base of the sculpture, built from sand brought in from Myrtle Beach, S.C., pounding and smoothing out the sand that had washed off the facade of the waist-up rendering of the chief executive.

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The sand sculpture was protected from above, and Mr. Obama’s face didn’t see too much damage. But the storm was so strong that its heavy winds blew the rain sideways, pelting the president’s right side and leaving the sand pockmarked and completely erasing his right elbow.

Democrats’ choice of Charlotte has drawn criticism from unions that don’t like North Carolina’s labor laws, and the state seems to be tilting away from Democrats politically.

The large Mount Rushmore-style sculpture drew comparisons to Mr. Obama’s 2008 convention in Denver, when he accepted his party’s nomination on a stage that looked like a Greek temple.