Final answer:
The major party in Britain until 1931 was the Liberal Party, known for promoting political reform and expansion of the electorate. It was challenged by the Conservative Party and eventually by the rising Labour Party. The Liberal Party's decline was marked by internal conflicts and changes in the political landscape.
Step-by-step explanation:
The major party in Britain until 1931 was the Liberal Party. In the 19th century, the foundation of Britain's modern political system was established as the Whig Party transitioned into the Liberal Party under the leadership of William Gladstone, and the Tories became the Conservative Party led by Benjamin Disraeli.
The Liberal Party, with strong 19th-century roots and significant contributions to political reform, experienced internal conflicts, particularly during World War I which exacerbated its vulnerability to the rising Labour Party.
Ultimately, the Liberal Party's influence waned after 1931 as the Labour Party took on a greater role in British politics. This period saw significant political changes, including the Great Reform Act of 1832 which expanded the electorate and the end of slavery in British territories in 1833, thanks to the liberalized parliament. The pattern of slow, steady liberalization was characteristic of the 19th century and set the stage for future political development.