RICHMOND, Calif. — Carlos Diaz broke the law when he crossed the border and took a job as an office janitor. But he's not about to break another by failing to pay his income tax.

"I've been talking to other people who've done it, and I want to follow the law," said Diaz, an undocumented immigrant from Guatemala who squirmed in his seat at a neighborhood tax preparer's office.

Tuesday is Tax Day, when millions of illegal immigrants find themselves collaborating with one federal agency — the Internal Revenue Service — while trying to avoid another — Immigration and Customs Enforcement.

They hope a track record of on-time payments will aid their citizenship applications, but critics who favor tougher enforcement of federal immigration rules say it's absurd for the government to work with people it should be tracking down and deporting. It legitimizes the presence of immigrants who are here illegally, critics say, and sends a mixed message about the country's interest in enforcing its own rules.

"The word schizophrenic comes to mind," said Marti Dinerstein, president of Immigration Matters, a research firm that advocates tighter immigration enforcement. "There is something fundamentally wrong about this."

The IRS created a nine-digit Individual Tax Identification Number in 1996 for foreigners who don't have Social Security numbers but need to file taxes in the U.S. But it is increasingly used by undocumented workers to file taxes, apply for credit, get bank accounts or even buy a home.

The IRS issued 1.5 million ITINs in 2006 — a 30 percent increase from the previous year. To obtain one, a person needs to submit to the IRS an application and a document that serves as proof of identity, such as a visa or driver's license. All told, the tax liability of ITIN filers between 1996 and 2003 was $50 billion. The agency has no way to track how many were immigrants, but it's widely believed most people using ITINS are in the United States illegally.

One number hints at the number of illegal immigrants having income taxes deducted from their paychecks.

In 2004, the IRS got 7.9 million W-2s with names that didn't match a Social Security Number. More than half were from California, Texas, Florida and Illinois, states with large immigrant populations, leading experts to believe they likely represent the wages of illegal immigrants. Even immigrants who use ITINs to file taxes are forced to make up a Social Security Number when they get a job.

Critics like Dinerstein say the process makes room for law violators, and in some cases, might endanger the country by allowing them to operate more freely.

"That's why people who are living here illegally rushed to get ITINS like they're chocolate candy," said Dinerstein. "It's a national security issue."

IRS spokeswoman Nancy Mathis said the ID numbers are issued to track a tax return's progress through the system, noting the tax code says nothing about whether foreigners filing taxes are here legally or not.

"It serves no other purpose," she said, "and was never intended to serve any other purposes."

Nor does the IRS share immigrants' personal information with ICE or any other agency, Mathis said.

Many banks now allow illegal immigrants to open an account with their ITIN, and Bank of America has a pilot program in Los Angeles that allows customers to use the numbers to sign up for a credit card. Others have created mortgage products for ITIN-bearing immigrants, including Citibank, which offers one in partnership with ACORN Housing Corp.