The challenges Mitch McConnell is facing from the left and right in his reelection campaign have at least one thing in common: Each opponent is making an issue of the wealth the five-term incumbent has accumulated in office. McConnell over the summer reported a net worth somewhere between $9 million and $36 million (lawmakers are required only to list their holdings in ranges) — a substantial leap over the six-figure sums he reported in his early Senate years and enough to make him the 11th-richest member of the chamber.
Campaign innuendo aside, there’s a simple explanation for how a fellow in public service gets rich: He marries into it. In 1993, McConnell, whose first marriage ended in divorce, wed Elaine Chao, an up-and-coming Republican operative then serving as the president of United Way of America. She’s also the eldest of Chinese-born shipping magnate James Chao’s six daughters. Her father built a fortune as founder and chairman of the Foremost Group, a privately held outfit based in New York City that moves iron ore, coal, scrap metal, and wheat around the world. And Chao père delivered the bulk of the couple’s wealth in a 2008 gift valued between $5 million and $25 million. The Chao family has also been a reliable supporter of McConnell’s campaigns and the Kentucky state party, contributing $391,000 over the course of his career.
The Republican leader may need to tap those resources again for his current electoral battles — though one in-law may be less inclined to answer the call: Elaine’s youngest sister, Angela, is married to billionaire Facebook investor Jim Breyer, a prominent VC at Accel Partners. He has contributed primarily to Democratic candidates, and with Mark Zuckerberg he is funding efforts to pass comprehensive immigration reform that McConnell opposed. Angela’s first husband, the late billionaire financier Bruce Wasserstein, was a committed supporter of Democrats, though Angela’s giving has been exclusively to Republicans so far.
Within McConnell’s marriage, the career support flows in both directions. When George W. Bush tapped Chao to head his Labor Department, she hired her husband’s longtime chief of staff and his communications director. These days, Chao serves as a fellow at the conservative Heritage Foundation (which is now run by McConnell antagonist Jim DeMint) and as a director of Michael Bloomberg’s family charity.
This story is from the April 7, 2014 issue of Fortune.