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: