Read this and answer the question. If the question is unanswerable, say "unanswerable".

The United States has become essentially a two-party system. Since a conservative (such as the Republican Party) and liberal (such as the Democratic Party) party has usually been the status quo within American politics. The first parties were called Federalist and Republican, followed by a brief period of Republican dominance before a split occurred between National Republicans and Democratic Republicans. The former became the Whig Party and the latter became the Democratic Party. The Whigs survived only for two decades before they split over the spread of slavery, those opposed becoming members of the new Republican Party, as did anti-slavery members of the Democratic Party. Third parties (such as the Libertarian Party) often receive little support and are very rarely the victors in elections. Despite this, there have been several examples of third parties siphoning votes from major parties that were expected to win (such as Theodore Roosevelt in the election of 1912 and George Wallace in the election of 1968). As third party movements have learned, the Electoral College's requirement of a nationally distributed majority makes it difficult for third parties to succeed. Thus, such parties rarely win many electoral votes, although their popular support within a state may tip it toward one party or the other. Wallace had weak support outside the South. More generally, parties with a broad base of support across regions or among economic and other interest groups, have a great chance of winning the necessary plurality in the U.S.'s largely single-member district, winner-take-all elections. The tremendous land area and large population of the country are formidable challenges to political parties with a narrow appeal.

Which party was anti-slavery?
Democratic Party