The 1964 United States Senate elections were held on November 3. The 33 seats of Class 1 were contested in regular elections. Special elections were also held to fill vacancies. They coincided with the election of President Lyndon B. Johnson by an overwhelming majority, to a full term. His Democratic Party picked up a net two seats from the Republicans. As of 2023, this was the last time either party has had a two-thirds majority in the Senate, which would have hypothetically allowed the Senate Democrats to override a veto, propose constitutional amendments, or convict and expel certain officials without any votes from Senate Republicans. In practice, however, internal divisions effectively prevented the Democrats from doing so. The Senate election cycle coincided with Democratic gains in the House in the same year.
The 1944 United States Senate elections coincided with the re-election of Franklin D. Roosevelt to his fourth term as president. The 32 seats of Class 3 were contested in regular elections, and three special elections were held to fill vacancies.
The 1912–13 United States Senate elections were held on various dates in various states. They were the last U.S. Senate elections before the ratification of the Seventeenth Amendment in 1913, establishing direct elections for all Senate seats. Senators had been primarily chosen by state legislatures. Senators were elected over a wide range of time throughout 1912 and 1913, and a seat may have been filled months late or remained vacant due to legislative deadlock. Some states elected their senators directly even before passage of Seventeenth Amendment. Oregon pioneered direct election and experimented with different measures over several years until it succeeded in 1907. Soon after, Nebraska followed suit and laid the foundation for other states to adopt measures reflecting the people's will. By 1912, as many as 29 states elected senators either as nominees of their party's primary or in conjunction with a general election.
The 1872–73 United States Senate elections were held on various dates in various states, coinciding with President Ulysses S. Grant's re-election. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1872 and 1873, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 3.
The 1898–99 United States Senate elections were held on various dates in various states. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1898 and 1899, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 1.
The 1902–03 United States Senate elections were held on various dates in various states. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1902 and 1903, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 3.
The 1904–05 United States Senate elections were held on various dates in various states, coinciding with President Theodore Roosevelt's landslide election to a full term and the 1904 House of Representatives elections. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1904 and 1905, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 1.
The 1884–85 United States Senate elections were held on various dates in various states, coinciding with the presidential election of 1884. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1884 and 1885, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 3.
The 1890–91 United States Senate elections were held on various dates in various states. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1890 and 1891, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 3.
The 1892–93 United States Senate elections were held on various dates in various states, coinciding with former Democratic President Grover Cleveland's return to power. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1892 and 1893, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 1.
The 1899 United States Senate election in Massachusetts was held during January 1899. Republican incumbent Henry Cabot Lodge won election to a second term easily.
The 1887 United States Senate election in Massachusetts was held during January 1887. Republican incumbent Henry L. Dawes was re-elected to a third term over opposition from within his own party, led by former Governor John Davis Long.
The 1857 United States Senate election in Massachusetts was held in January 1857. Incumbent Charles Sumner was re-elected to a second term in office as a member of the Republican Party. Sumner was elected in 1851 by a single vote after twenty-five inconclusive ballots by a coalition of Free-Soil and Democratic legislators. He had since become a founding member of the Massachusetts Republican Party.
The 1855 United States Senate special election in Massachusetts was held during January 1855. Henry Wilson was elected to fill the remainder of the term left vacant by the resignation of Edward Everett.
The 1865 United States Senate election in Massachusetts was held during January 1865. Incumbent Republican Senator Henry Wilson was re-elected easily to a second term as a member of the Republican Party.
The 1871 United States Senate election in Massachusetts was held on January 17, 1871. Incumbent Republican Senator Henry Wilson was re-elected easily to a third term as a member of the Republican Party. Wilson would not finish the term, since he was elected Vice President of the United States in 1872.
The 1873 United States Senate special election in Massachusetts was held in March 1873 to fill the vacancy left by Senator Henry Wilson, who resigned to become Vice President of the United States. George S. Boutwell won the election.
The 1877 United States Senate election in Massachusetts was held in January 1877. Incumbent Republican Senator George S. Boutwell, who had won a special election for the remainder of Henry Wilson's term, was defeated by reformist U.S. Representative George Frisbie Hoar.
The 1883 United States Senate election in Massachusetts was held in January 1883. Incumbent Republican Senator George Frisbie Hoar was re-elected to a second term in office despite a serious challenge from Democrats and members of his own party.
The 1889 United States Senate election in Massachusetts was held in January 1889. Incumbent Republican Senator George Frisbie Hoar was re-elected to a third term in office with no serious opposition.
