Two women are worth noting when you're talking about the first woman to serve in the United States Senate: Rebecca Latimer Felton and Hattie Caraway.
Rebecca Latimer Felton was the first woman to serve on the U.S. Senate, but she served for only one day. Her appearance there began as a political maneuver.
In 1922, Thomas Hardwick, Governor of Georgia, wanted to run for a Senate seat in his state. However, he had been opposed to the Nineteenth Amendment, which gave women the right to vote in 1920, and he knew he wouldn't be popular among the women of his state who were now registered voters. He needed a way to get in their good graces if he wanted to win.
His opportunity came when Senator Tom Watson died on September 26, 1922. As Governor, Hardwick was charged with filling that vacant Senate seat until the next election (which he hoped to secure for himself). Hardwick chose Rebecca L. Felton, a prominent suffragist — and white supremacist.
The move was pure political posturing. Congress was out of session and was not expected to convene again until after the election, so Felton would never be properly seated.
Despite his chicanery, Hardwick lost the election in November to Walter George. George did not ignore Felton's appointment, however, and when Congress convened on November 21, 1922, he stepped aside and allowed Felton to take the oath and become the United States' first female Senator (and, at over 87 years old, the oldest freshman Senator ever sworn in), a position she held for the next 24 hours.
Ten years would pass before the U.S. Senate would see another woman serving in its chambers. After Senator Thaddeus Caraway of Arkansas died suddenly late in 1931, Arkansas Governor Harvey Parnell appointed Caraway's widow Hattie Caraway to fill the vacant seat. She was sworn in on December 9. In January of 1932, she easily won a special election to serve the remaining months of her husband's term, becoming the first woman to be elected to the U.S. Senate.
In May of that year, she surprised many in Arkansas politics, who thought she would step aside at the end of that term, by announcing that she would run for a full term in the 1932 election. With backing from Louisiana politician Huey Long, she won that election, becoming the first female full-term Senator elected to Congress.
She won a second term in 1938, but placed fourth in the Democratic primary of 1944.