Kern won election to the United States Senate in 1910, becoming a progressive ally of President Woodrow Wilson. He was elected Chairman of the Senate Democratic Caucus and helped pass several major pieces of legislation, including the Clayton Antitrust Act, the Revenue Act of 1913, and the Federal Reserve Act. He also introduced the Kern Resolution, which led to the investigation of conditions in coal mines, and supported passage of the Seventeenth Amendment. He was defeated for re-election in 1916, losing to Republican Harry Stewart New, and Kern died the following year.