In a boost for supporters of a major immigration reform bill now on the Senate floor, the Congressional Budget Office on Tuesday reported that the plan would reduce the nation's budget deficit over the next two decades, as backers said it showed the "economic imperative" to approve immigration law changes.

"The nonpartisan Congressional Budget Office has confirmed what supporters of comprehensive immigration reform have long understood—that reforming our Nation’s broken immigration system makes economic sense," said Sen. Patrick Leahy (D-VT).

"CBO says immigration reform will melt the deficit," said Sen. Chris Murphy (D-CT). "Nice when the right thing to do also saves you money."

"The CBO has further confirmed what most conservative economists have found: reforming our immigration system is a net benefit for our economy, American workers and taxpayers," said Sen. Marco Rubio (R-FL).

"Immigration reform would reduce the deficit by nearly $1 trillion over two decades," the White House proclaimed on Twitter.

Republicans were quiet about the CBO review, once again proving what I refer to as the "CBO Rule" - if the CBO issues a cost estimate that you like, you talk about it in glowing terms.

But if the CBO issues a cost estimate that doesn't go your way, then you either ignore it or say it's a bunch of fiscal rubbish.

"The bill’s drafters relied on the same scoring gimmicks used by the Obamacare drafters to conceal its true cost from taxpayers and to manipulate the CBO score," said Sen. Jeff Sessions (R-AL), who has led the fight against the immigration bill on the Senate floor.

Let's take a look at some of the specifics in the report, which you can read for yourself on the CBO web site.

+ CBO estimates by 2012, the immigration reform law would mean a net increase of 10.4 million people living in the U.S.

+ CBO predicts immigration reform would result in a drop in the budget deficit of $197 billion over ten years

+ Federal spending would go up under this bill by $262 billion over ten years, but that would more than be offset by a $459 billion increase in tax revenues.

+ CBO belives the immigration bill would spur economic growth, raising GDP by 3.3% in 2023 and 5.4% in 2033

The release of the CBO report came soon after the Senate defeated two more Republican plans to tighten border security provisions in the bill.

Both a plan to force the construction of 700 miles of border fence and to institute a biometric entry/exit system before people in the US illegally could begin a path to citizenship were easily defeated; neither plan could muster 40 votes.

After just a few votes on GOP border security plans, it seems clear that supporters of immigration reform have more than a majority in the Senate - but still aren't sure there are 60 votes there to force final action on the measure next week.

But they're certain to now add this CBO argument to their arsenal during debate on the Senate floor.