Rich in history and treasured by tourists, schoolchildren and parishioners alike, California's 21 Spanish missions are nevertheless in peril.

Built in the late 18th century, the aging structures and the artifacts they hold are in dire need of repair and upgrades, but a debate is smoldering over whether taxpayers should foot the bill.

If a measure passed by the U.S. Senate becomes law, $10 million in federal funds would be earmarked for mission restoration — a violation, critics say, of the separation of church and state, since most of the missions are still owned by the Catholic Church and continue to hold Mass.

"If people who are religious and of that particular faith do not want to find ways to privately raise these funds, it's not up to the American taxpayer to bail them out and rebuild those religious structures," said Barry W. Lynn, executive director of the advocacy group Americans United for Separation of Church and State (search).

But the California Missions Foundation (search) says that while most missions do hold Sunday services and have active parishes, they are primarily used for secular and educational purposes. Knox Mellon, executive director of the foundation, said he believes missions should get tax money since they're historic monuments, and his organization will make sure the money won't be used to support religion.

The Senate passed the bill before they left earlier this month. But in an attempt to answer the church-and-state concerns, an amendment was attached saying the money couldn't be given out unless the Justice Department issued a finding that it doesn't violate the First Amendment (search).

The latest version of the bill must still pass the House, which it is expected to do when Congress reconvenes for a lame-duck session after the Nov. 2 general election. If it passes, it will then go to the president.

Click on the video box at the top of this story to watch a report by FOX News' Claudia Cowan.