“We are where gays were at the time of Stonewall,” said Lori Lipman Brown, the director of the Secular Coalition, referring to the 1969 riot in Greenwich Village that was the birth of the gay rights movement. “And the thing we have in common with gays back then is that day to day you’re hidden. If you make the decision to come out, you’re treated very badly.”

“We should have a base of at least 30 million Americans to work with,” Ms. Brown continued. “And yet those who are active are a much smaller percentage. We’re probably looking at just a few hundred thousand active participants. It’s hard to even quantify.”

The situation in Colorado offers a case history of some particular obstacles to organizing the secular constituency.

Amendment 48 is exactly the kind of starkly drawn measure that political consultants refer to as an “emotional trigger” for prospective voters. Yet such a ballot item also demands the tactile, personal campaigning known in the trade as “retail politics.” A voter in Colorado this year faces 14 ballot initiatives as well as choices for president, senator and representative, and it will take no small amount of motivation and training to find, much less vote on, Amendment 48.

Polls since the summer have shown the amendment trailing, with about 40 percent of likely voters in favor, about 50 percent opposed and about 10 percent undecided. While the umbrella group for the amendment’s foes, the No On 48 coalition, includes some secular organizations, many of its most active volunteers come from issue-based organizations like Planned Parenthood or liberal religious denominations like mainline Protestants and Reform Jews.

One problem with turning out the atheist vote is finding it. Atheists do not reside visibly in certain neighborhoods like blacks or Hispanics or gay men and lesbians. They do not turn up on the databases of professional associations like doctors or lawyers. And as nonbelievers, they axiomatically do not come together for worship.

“It’s harder for them to organize,” said Brian Graves, 27, an organizer for No On 48, “because they don’t have something to congregate around.”