FACTBOX: Key U.S. ballot initiatives and the 2008 election

(Reuters) - Ballot initiatives allow voters to directly decide whether to change state laws or amend state constitutions.

The proposals are placed on the ballots in presidential or congressional election years. Following are results of some of the initiatives on Tuesday's ballot.

GAY MARRIAGE/GAY RIGHTS

CALIFORNIA - An amendment to the state constitution recognizing marriage as only between a man and a woman was leading, 52 to 47 percent, with 36 percent of precincts counted late on Tuesday night.

The amendment would eliminate gay marriages, overriding a ruling by the California Supreme Court in May, and was viewed as a key battle in the nation's "culture war" because the state is seen as a trend setter for the rest of the country.

Voters in Florida and Arizona also passed gay marriage bans.

ARKANSAS - Voters approved a ballot measure banning unmarried couples from adopting or serving as foster parents that was seen as a salvo against same-sex marriage and gay couples.

AFFIRMATIVE ACTION/CIVIL RIGHTS

NEBRASKA - Voters passed a constitutional amendment banning state and local governments from giving preferential treatment on the basis of race, sex, ethnicity or national origin.

A similar initiative in Colorado was too close to call late on Tuesday night.

ABORTION

SOUTH DAKOTA - Voters rejected a ban on abortion except in cases of rape, incest or if the pregnancy threatens the mother's health or life.

COLORADO - Voters rejected by about a three to one margin a measure that would have made abortion the legal equivalent of murder by defining human life as beginning at conception

ANIMAL RIGHTS/WELFARE

CALIFORNIA - A ballot initiative requiring more humane treatment of farm animals appeared headed for a landslide victory, winning 62 percent of the vote with 35 percent of precincts counted late on Tuesday night.

Proposition 2 seeks a constitutional amendment that requires more living space for some farm animals, including pregnant pigs and egg-laying hens. Opponents argued that the measure could cripple California's $300 million egg production industry.