You come up out of the subway in midtown Manhattan and you’re not quite all the way to your destination.

Where’s the bus? You need a transfer.

President Trump seemingly found himself in the same position as New York commuters these days. He’s getting some money for his border wall. But he’s not all the way there yet. The border security package approved by Congress grants the President $1.375 billion for a physical barrier. Mr. Trump wants $5.7 billion.

In other words, Mr. Trump needs to make it down to the Financial District, but he’s still stuck up in the South Bronx. The President needs a transfer. And in the case of the border security measure, something in the Congressional/White House/appropriations world known as “transfer authority.”

House/Senate border security conferees crafted a $49 billion package for the Department of Homeland Security. Article I, Section 9 of the Constitution dictates that “No Money shall be drawn from the Treasury but in Consequence of Appropriations made by law.”

In other words, Congress wields the ultimate power of the purse in American government. Article I, Section 7 of the Constitution grants the President authority to either sign or veto bills.

That’s it.

Article I, Section 1 of the Constitution declares that “All legislative Powers herein shall be vested in a Congress of the United States. That means only the House and Senate control legislative text and dictates how it works. Not the executive branch.

That is, unless the White House uses “transfer authority,” sometimes called “reprogramming.”

Here’s how it works:

There are 12 individual spending bills. Think of each as a silo containing various spending. One for Commerce, Justice & Science programs. Another for Defense. One for the Department of Homeland Security.

Let’s say we’re talking about the Department of Homeland Security appropriations bill. Loaded into each bill are multiple “accounts.” Congress steers money to various programs and projects in each account.

Imagine an “Account A” and “Account B” in the DHS measure. The administration may decide it needs more money for Account B. So, it wants to “reprogram” the money from Account A to Account B. Such a maneuver is allowed. But the gambit entails a sign-off by the top members of House and Senate Appropriations Committees and DHS Appropriations Subcommittees.

Now, let’s go to a higher level of chess.

Congress approves the DHS appropriations measure and the defense appropriations bill. The President signs both pieces of legislation into law. But down the road, the administration decides it has a need under the aegis of DHS and wants to transfer money from the Pentagon. That too requires the blessing of top appropriators. And, if the President wishes to retool the defense appropriations bill, he also must earn approval from leaders of the House and Senate Armed Services Committee.

They’re not known as appropriators, in Congressional-ese, but “authorizers.”

This tactic starts to test the Constitutional limits of Article I, Section 9, granting Congress the authority to spend money how it sees fit. But what happens if administration starts to shift money Congress appropriations bills? This definitely pushes the limits of what is generally accepted and could run afoul of Article 1, Section 9. There would be a question as to which branch of government really has control over spending. Lawmakers from both parties may balk at this.

Also, Trump only has $750 million in transfer authority available within the DHS bill. So, he’d have to rob Peter to pay Paul, moving money between appropriations bills. Moreover, all of those programs from which the President wants to shift money have supporters in Congress from both parties. Those lawmakers will bray if the money touches their pet program. The howls are even more dire if the President does it without Congressional dispensation.

“The President is doing an end-run about Congress about the power of the purse,” said House Speaker Nancy Pelosi (D-CA).

“This is a terrible power grab, unprecedented in American history and would really upset the balance of power” added House Judiciary Committee Chairman Jerry Nadler (D-NY).

House Appropriations Committee Chairwoman Nita Lowey (D-NY) said that lawmakers would respond if they think the President abused executive authority. Lowey hinted that the Appropriations Committee may add strictures to prevent the President from pushing things too far the next time.

“That sounds very interesting,” said Lowey when asked about the prospects of trying to harness President Trump. “If the President is responding inappropriately, I hope we can respond in a bipartisan fashion.”

Sen. Patrick Leahy (D-VT), the top Democrat on the Senate Appropriations Committee, observed that the President “might get away with it once.” But Leahy added that lawmakers would work to protect their Congressional prerogatives.

Senate Majority Leader Mitch McConnell (R-KY) is an appropriations veteran himself, known for guarding Senate traditions and customs. Yet McConnell defended the President’s right to trod what could be extra-Constitutional turf.

“He ought to feel free to use whatever tools he wants to use to secure the border. I would not be troubled by that,” said McConnell.

That said, there are limits on how much money the President can mine from various appropriations bills. The DHS bill alone offers a threshold of but $750 million (that’s with an “m,” as in “Mary”) in funds available for a transfer.

I wasn’t very good at arithmetic in Mrs. Sherman’s 7th-grade math class, but adding $750 million to $1.375 billion for a wall is only a little more than $2 billion. That’s far short of the $5.7 billion in wall funding Mr. Trump pushed for last year.

Moreover, each of the spending bills bear what are called “sequestration caps.” In 2011, Congress approved the Budget Control Act (BCA). The measure set automatic limits on how much money is available in each of the 12 spending bills. Sequestration curbs how much Congress can spend in each area. You can’t just add on extra money. That creates a problem if the President begins to raid one bill to boost another – to say nothing of the impact on various spending programs. That’s why some lawmakers will screech if Mr. Trump’s push for the wall interferes with their special programs.

But, there really wasn’t much Trump could do unless he wanted to spark a government shutdown.

McConnell felt the President burned him back in December. McConnell had indications from the White House that the President would sign the interim spending package to avoid a government shutdown before Christmas. The Senate passed the bill. Then the President balked.

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As we have reported in this space, McConnell’s favorite expression is that “there’s no education in the second kick of a mule.” No fool he, McConnell made sure to lock in the President. McConnell came to the Senate floor announcing he secured Mr. Trump’s assurances that he would sign the spending package. That would bind the President to stand by his word. By the same token, McConnell also indicated President Trump would declare a national emergency.

One must dig deep into the Congressional archives to find a similar instance where a Congressional leader spoke on the President’s behalf on an issue of such gravity.

The House adopted the package 300-128. With 428 members voting, the magic number for passage was 215. Democrats scored 213 yeas. Democrats only needed the assistance of two Republicans to pass the bill. Otherwise, Democrats could have done it on their own.

House Majority Whip Jim Clyburn (D-SC) said he leaned on House Minority Whip Steve Scalise (R-LA) to provide at least 70 Republican ayes. Scalise delivered 87. A big number. But one would think that McConnell announcing that the President would sign the bill and declare a national emergency would actually drive the number of GOP yeas higher. Consider how low that Republican number could have been had there not been a guarantee from the President – via his surrogate spokesman from the Bluegrass State?

Regardless, the bill achieved veto-proof margins in both bodies. 83 yeas in the Senate. 300 in the House. Both tallies are well above the two-thirds threshold required to override a presidential veto. The House and Senate passed the bill. And now they can argue about what latitude the President has once Congress finishes its work.