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. Proposed amendments to ban gay marriage in several states galvanized the Republican Party's conservative Christian base and played a role in President George W. Bush's 2004 re-election victory.

Following are some of the initiatives that have qualified to be on the ballot. They could have an impact on Tuesday's White House election.

OVERVIEW:

- According to Ballotwatch, a University of Southern California institute that monitors such trends, there are more than 150 ballot propositions on the November ballot in 36 states.

GAY MARRIAGE/GAY RIGHTS:

CALIFORNIA - Voters will be asked to amend their state constitution to recognize marriage as only between a man and a woman, a bid to override a court ruling allowing same-sex unions. Similar proposals will be on the ballot in the battleground states of Arizona and Florida.

The California amendment is 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. Experts don't expect it to affect the presidential race in California because the state is solidly Democratic.

But Florida is a closely contested state and the measure there could make a difference as it energizes both parties' base.

ARKANSAS - A measure would change state law to ban unmarried couples from adopting or serving as foster parents.

AFFIRMATIVE ACTION/CIVIL RIGHTS

COLORADO - A proposed constitutional amendment will be on the November ballot in Colorado that says: "The state shall not discriminate against or grant preferential treatment to any group or individual on the basis of race, sex, color, ethnicity or national origin in the operation of public employment, public contracting or public education."

A similar proposal is on the ballot in Nebraska.

ABORTION:

SOUTH DAKOTA - Voters will be asked to ban abortion except in cases of rape, incest or if the pregnancy threatens the mother's health or life.

ANIMAL RIGHTS/WELFARE:

CALIFORNIA - Proposition 2 will seek a constitutional amendment that requires minimum living space for farm animals, including pregnant pigs and egg-laying hens.

Egg producers say the measure, if passed, could drive them out of business. Egg production is a $300 million industry in California.

ENERGY/ENVIRONMENT

CALIFORNIA - Voters will be asked to authorize up to $5 billion in incentives to buyers of high fuel economy and alternative fuel vehicles. It would also approve incentives for developing renewable energy technologies and fuels.

(Sources: Reuters, California Secretary of State, Florida Department of State, South Dakota Secretary of State, Family Research Council, American Civil Rights Institute, Ballot Watch, National Conference of State Legislatures)

(Compiled by Dan Whitcomb, Ed Stoddard and Tim Gaynor; editing by Todd Eastham)