One of the key selling points for amnesty is an alleged positive benefit for Social Security. Media Matters, the Center for American Progress, Senator Marco Rubio (R-FL), and Senator Harry Reid (D-NV) are among the prominent figures claiming the Senate’s amnesty bill will benefit Social Security. And according to the “intermediate” projections from the Social Security Administration (SSA), they are right.

Being aware that the Social Security Administration’s intermediate projections are frequently off-target, page 16 of the SSA’s report shows Social Security will benefit from the Senate’s immigration bill by $427 billion over the next 74 years. This is a total of 4.5% of the projected unfunded liabilities for the program over that time – not an insubstantial number.

True to Washington form, the report hides a key fact. At no point is the amnesty portion of the Senate’s bill differentiated from the rest of the analysis. The report lumps in the high-income legal immigrants with low-income illegal immigrants who tend to take far more out of the system than they add. Thus, amnesty looks far better for Social Security than it actually is.

Backers of the Senate bill are similarly misleading – while 75 years is where most long-term actuarial estimates end, the Senate bill’s supporters are looking at costs 10 and 20 years down the road. As Byron York’s analysis of the Senate bill’s impact on Social Security points out, this is disingenuous. This is the case for two reasons: first, the median illegal immigrant was 36 years old in 2011. This means most illegal immigrants won’t be touching Social Security (or Medicare) for at least three decades.

Second, much of the cost of the legalization of unauthorized immigrants will be seen at the state and local level. To claim the amnesty bill will help America’s finances while ignoring the impact on non-federal governments is disingenuous at best.

In the end, adding an influx of workers to pay taxes will, help Social Security in the short-term. But the refusal of the SSA to break down the different impacts of high-skilled, high-income legal immigrants and the low-income illegal immigrants misleads on exactly what amnesty will cost. As such, months after the Senate’s Gang of Eight first sat down, and nearly two weeks after the Senate passed its version of immigration “reform,” nobody knows what amnesty will actually cost.