WASHINGTON — At first, he had to worry about a remote piece of land in Arkansas that no one wanted. Then there were billing records that went missing before mysteriously reappearing in the White House. And of course there was the blue dress.

Today, the object of concern for David E. Kendall is a tiny thumb drive that sat in a safe at his law firm until a couple weeks ago before attracting the attention of Congress, the F.B.I. and the news media. Once again, the whirlpool of Washington politics has arrived at Mr. Kendall’s doorstep as he defends perhaps the world’s most famous client.

For more than 20 years, Mr. Kendall has been on the front lines for Bill and Hillary Rodham Clinton as their personal lawyer, battling investigators and litigants in the superheated environment where law and politics meet. From Whitewater to impeachment, he has waged legal warfare to keep the Clintons’ political careers on track. So as Mrs. Clinton faces questions about her use of a personal email server as secretary of state, no one is surprised she turned to Mr. Kendall.

The latest furor has put Mr. Kendall under a spotlight in a way that discomfits the tight-lipped and camera-shy lawyer. From Mrs. Clinton’s foes come public questions about why he had the thumb drive containing her email and whether he secured it properly. From Mrs. Clinton’s friends come private questions about whether he has managed the situation effectively and whether he should be more outspoken to protect a Democratic presidential candidate leading in the polls.