WASHINGTON (AP) — As the U.S. census nears its final stages, the government is preparing for possible debacles that could derail its $15 billion head count, from mass identity theft and lawsuits to homeowners who refuse to answer their doors.

Census Bureau documents, obtained by The Associated Press, underscore the highly fragile nature of the high-stakes population count before the government dispatches some 700,000 temporary workers to visit homes, beginning in May.

The preparedness efforts are not entirely new. Previous censuses had contingency plans in place, at least conceptually, and the Census Bureau has never failed to meet its constitutional mandate of delivering population counts by Dec. 31 each decennial year.

But this is the first time the Census has detailed — in 300 pages of internal documents released under the Freedom of Information Act — specific risks to the once-a-decade government count. It’s part of the bureau’s approach to handling threats that could undermine accuracy, omit large segments of the public or add to already ballooning costs.

Many of the documents proved telling, even with portions redacted or withheld for security reasons.

‘‘Considering the volume of data that the Bureau of Census gathers during the census, some loss of confidential data is bound to occur,’’ one document bluntly states. Citing past missteps, such as the loss of work laptops by census employees in 2006, it details a rapid-response effort that includes notification of authorities, if appropriate, as well as free credit monitoring for potential identity theft victims.

One document says the ‘‘No. 1 concern’’ could be a refusal by immigrants to participate.

Placing a cap on costs if immigrants try to evade the count, the response plan notes that a census worker will attempt to visit a home six times at most — or fewer, if a resident makes clear he won’t cooperate — before the worker questions neighbors to get the information. If that fails, the Census Bureau will statistically impute data based on characteristics of neighboring households.

In 2000, imputation, a statistical method that was not part of previous court battles over statistical sampling, increased the U.S. population by 1.1 million, particularly among urban racial minorities who would have been missed by a head count. Census Bureau director Robert Groves has ruled out sampling but not other statistical methods.

Another risk being monitored by the Census Bureau is the possibility of a conservative boycott following recent rhetoric, including one blogger’s threat to pull out a shotgun to scare away census workers. The White House condemned the remarks Tuesday, and the bureau said it remains on the lookout for signs of a boycott or other trouble. Conservatives who refuse to participate may also be counted by way of neighbor questioning or statistical imputation.

‘‘With these things, anything can turn on a dime, implode and impact our ability to recruit staff and gain cooperation,’’ Arnold Jackson, the bureau’s associate director for the decennial census, said in an interview. ‘‘We also remain terribly concerned about safety.’’

He said the bureau has tightened security procedures and boosted targeted advertising to specific groups, including a public service announcement released this week featuring President George W. Bush’s former political adviser, Karl Rove. Still, while there have been anecdotal reports that conservatives may fill out only the number of people in their households, Jackson says there has been little sign of incomplete census forms received so far.

Jackson said the Census Bureau will tap into its reserve fund of roughly $7 million for ad buys in low-response regions, which he identified as having predominantly ‘‘non-English speaking households, areas of heavy concentration of minority groups, such as African-American and Latino, and urbanized areas.’’

Currently, the mail participation rate for the 2010 census is 63 percent, and officials remain cautiously optimistic they will be able to match, if not top, the 2000 mail-back rate of 72 percent by the time the Census Bureau winds down its mail-in operation in late April. From May to July, census takers are sent to homes that do not mail back forms.

Other potential risks include:

—Information technology breakdowns. A key software system used to schedule, deploy and pay census takers is full of defects, and its ability to handle a massive payroll of more than 600,000 temporary employees who begin work in May remains in doubt. Robert Goldenkoff, a director at the Government Accountability Office, believes the census could face serious delays if IT problems aren’t fixed soon; the Census Bureau says it’s dealing with the problem.

—Lawsuits. Census officials are calling it ‘‘relatively certain’’ that they will be sued by at least one state that just misses gaining an additional U.S. House seat. The state will probably move quickly ‘‘in order to potentially reverse the December 2010 announcement’’ of the official head count figures, according to documents. Officials said litigation could involve outreach to non-English speaking communities or census methodology, such as the count of prisoners, non-citizens or religious missionaries overseas.

—Mass retirements. A significant number of Census Bureau employees are now eligible to retire, and the 2010 count could be in peril if there is a mass exodus during peak census periods. So far, however, there has been no sign of that, according to documents. In the meantime, the bureau is developing detailed plans to make the learning curve shorter for newly hired employees.