(CNN) It's a surprising fact that's often overlooked in the immigration debate.
Undocumented immigrants pay billions of dollars in federal taxes annually, between tax returns filed and taxes deducted from paychecks, experts estimate.
Here's a look at why -- and how -- that's happening:
• It shows they're complying with federal tax laws.
• It can help them demonstrate "good moral character" if they later have an opportunity to legalize their immigration status.
• Tax return records could be used to document work history and presence in the US, steps that may help them be eligible for legal immigration status in the future if lawmakers pass immigration reform.
Critics of illegal immigration have long argued that undocumented immigrants who pay taxes are able to do so because they're using stolen Social Security numbers. But millions of federal tax dollars are paid every year by people who don't have Social Security numbers at all.
Instead, they file using what's known as an Individual Taxpayer Identification Number (ITIN).
The Bipartisan Policy Center, a Washington-based think tank, notes that "most experts believe the vast majority of tax returns filed with ITINs today are filed by undocumented immigrants."
Some noncitizens who legally immigrated to the United States also pay taxes using this method.
In 2015, according to the IRS, 4.35 million tax returns were filed using ITINs, accounting for more than $13.6 billion in taxes.
In addition to tax return filings, officials have estimated that undocumented immigrants also contribute billions to Social Security annually through payroll tax deductions. In 2010, for example, the Social Security Administration estimated that payments from unauthorized workers accounted for about $12 billion in tax revenue for Social Security.
A few years ago, one Arizona college student's posts about her experience paying taxes drew attention to the issue.
This year advocates are once again taking to Twitter to point out that undocumented immigrants are paying taxes.
Jose Antonio Vargas, the Pulitzer Prize-winning journalist-turned-activist who's turned his struggles as an undocumented immigrant into a platform for advocacy, tweeted that he was finalizing his taxes over the weekend.
"Yes," he wrote, "undocumented immigrants are helping fund the very system that detain and deport us."