Upon his election to the United States Senate, Brewster built a staff that included two recent college graduates: Nancy D’Alesandro, who would later become Nancy Pelosi, the Speaker of the U.S. House of Representatives, and Steny Hoyer, who would later become Majority Leader in the U.S. House of Representatives. 

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Brewster was the only Democratic United States Senator south of the Mason-Dixon Line to cosponsor the Civil Rights Act of 1964 and the Voting Rights Act of 1965 and support the Fair Housing Act of 1968. Brewster also sponsored the successful legislation, signed into law by the President, that created the Assateague Island National Seashore. 

After President Kennedy’s assassination, President Lyndon B. Johnson asked Senator Brewster to enter Maryland’s Democratic Presidential Primary in 1964 to oppose the racist segregationist Alabama Governor George C. Wallace, who had entered Maryland’s primary in hopes of winning, which, in turn, would help to defeat the Civil Rights Act of 1964. Brewster defeated Wallace, which became the front page leading story of every major newspaper in America and led to the passage of the Civil Rights Act of 1964 weeks later.

After the 1964 Presidential primary, Brewster, who felt that he should have beaten Wallace by an even greater margin, began to drink more heavily, and his alcoholism began to impact his performance as a United States Senator. Brewster’s strong support of civil rights angered many Marylanders who did not share his passion. He was booed, cursed, and spat upon by pro-segregationist Marylanders. Additionally, Brewster continued to support the Johnson Administration’s policy in Vietnam, believing that, if we were there, the United States should commit the resources necessary to win. The American public had already turned against the war. With Brewster’s 1967 divorce from Carol Brewster and his quick remarriage to Anne Bullitt, Brewster’s political problems continued to mount. In 1968, Brewster was defeated for reelection to the United States Senate by one of his best friends, Congressman Charles McC Mathias Jr., who had been his classmate in law school, served in each other’s weddings, and Mathias was godfather to Brewster’s oldest son, Dan Jr.

Following his election loss, debilitated by alcoholism, and committed to an Irish hospital, Brewster learned that he had been indicted and accused of taking bribes by a former member of his staff while serving in the Senate. Brewster’s legal battle lasted for six long years which included two trips to the United States Supreme Court, a trial, conviction, successful appeal, charges reinstated, and ultimately ended with Brewster pleading nolo contendere to one count of accepting an unlawful gratuity “without corrupt intent.” He was fined $10,000, and the Maryland Court of Appeals ruled that he could keep his license to practice law since the one count was “without corrupt intent” and did not involve “moral turpitude.”

Brewster’s life had hit rock bottom, suffering from alcoholism, his reputation and finances shattered, his family and friends estranged, and seemingly nothing to live for. Brewster, however, was not done, and he found the strength to overcome his “sea of troubles.” He went to rehab, rebuilt his life, started a new family, became a doting husband and father, served on and chaired numerous boards and commissions, and found happiness as he rebuilt a life of meaning and purpose.