WASHINGTON — As the well-connected wife of the longest-serving congressman in American history, Debbie Dingell has long been her husband’s fiercest protector. So when Representative John D. Dingell Jr., the famously irascible Michigan Democrat, announced in February that he would retire after nearly 60 years, he gave her a tender shout-out.

“The lovely Deborah and I are simply coming home,” Mr. Dingell said.

Few believed him, and with good reason: “the lovely Deborah,” a former auto industry lobbyist and General Motors executive, promptly ran for her husband’s seat and won, positioning herself as heir to a political dynasty that dates to Franklin Roosevelt.

Today she is Representative-elect Dingell, making her own history as the first woman to succeed a living spouse in Congress.

But as she prepares for life as a freshman in a shrunken Democratic minority, Mrs. Dingell, who turned 61 on Sunday, is also facing a reality she calls “bittersweet.” Just as her own career takes off, her 88-year-old husband, once one of the most powerful men in Washington, is leaving office because of his declining health.