Final answer:
Liberals in the United States are traditionally associated with the Democratic Party, which embodies center-left democratic liberalism, especially post-1980s political polarization.
Step-by-step explanation:
Liberals in the United States have traditionally been associated with the Democratic Party, particularly in modern times as the Democratic and Republican parties became highly polarized. The Progressive Party has, at times, been a home for those with liberal tendencies, but it generally represented a specific era and set of reforms rather than being synonymous with liberalism overall. Additionally, the conflation of liberals with the Green Party or the Conservative Party is misleading. While it is common for liberals to align with environmentally-friendly policies like those of the Green Party, it does not define their ideological home. Historically, the Conservative Party does not align with liberal values. Therefore, when asking which party liberals tended to belong to, the Democratic Party is the accurate answer, embodying center-left democratic liberalism, especially in the context of late 20th and early 21st-century U.S. politics.