Backers of Ralph Nader have turned in almost 15,000 signatures to get his name on the presidential ballot here. That’s enough, if the signatures are valid. Challenges are likely. The Democrat Party has challenged similar petition efforts in Illinois. A spokesman for the Missouri Democratic Party says its officials are taking a close look at the petitions in Missouri but have made no decision. Democrats fear Nader will help President George W. Bush by drawing votes from the Democrat nominee. Nader was on the ballot four years ago. His vote total did not change the outcome in this state.