Washington (CNN) Republicans like Jeb Bush and Scott Walker are criss-crossing early primary states, doing TV interviews on their policy positions and meeting with campaign donors.

But they are not running for president. Not technically, anyway.

That's why when Jeb Bush accidentally said last week he was running, he quickly slipped in a correction - "if I run."

Of course he's running, but there's one overriding reason for Bush to be coy: money.

The dirty little secret of the 2016 campaign is that would-be candidates like Bush and Walker in particular can use super PACs, campaign accounts that allow unlimited contributions, to raise millions of dollars as long as they aren't official candidates. Until someone like Bush, Walker or New Jersey Gov. Christie formally declares for president, they are legally permitted to personally ask for money for a super PAC that will ultimately benefit their campaign. But once they formally acknowledge their candidacy, a legal wall goes up between the candidate and the super PAC that supports them.

This is the confusing new world of campaign finance in the era after the Supreme Court's Citizens United decision in 2010 removed most political spending limits on corporations and unions. Now, until they're an official candidate, politicians can accept millions from a single donor, though Bush has set a self-imposed limit of $1 million for his super PAC.

The quiet goal for Bush, Walker and, to a lesser extent, Christie is to raise as much money as possible for as long as possible without the fundraising limits that come with being a declared presidential candidate.

Different rules for senators

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Legally, a federal office holder -- such as a senator -- is always a candidate for federal office, and therefore already bound by federal fundraising limits in a presidential run: $5,400 per maximum per donor, for the primary and general election combined. Super PACs that support them can raise money to use on their behalf, but they cannot coordinate in the same way Bush, Christie and Walker are.

So in the ever important money race, Cruz, Paul and Rubio are going to be judged by how much they raise inside a traditional Federal Election Commission filing quarter. This current quarter lasts from April 1 to June 30th. That is a major reason most GOP senators running for the White House declared at the beginning of April -- in order to squeeze as much fundraising time in as possible.

The super PAC phenomenon was born after the Supreme Court's Citizens United decision, as well as other court and FEC rulings. But it has taken a while for campaign operatives to figure out the best way to use the rules to their advantage.

Aides to the campaigns-in-waiting of Bush and Walker in particular seem to have done just that.

"Anyone with smarts will hire very knowledgeable people that can help them navigate very tricky waters and take advantage of everything they can while still remaining on the good side of the law," said Ana Navarro, a Republican operative who supports Jeb Bush.

Being on the good side of the law means once a candidate officially declares, their campaign can no longer strategize -- or even communicate -- with its super PAC, which is generally run by a long-time adviser or aide.

In the case of Bush, Mike Murphy will run the "Right to Rise" super PAC. Walker's long-time chief of staff Keith Gilkes, who also ran Walker's 2010 gubernatorial race, is heading up the Walker Super PAC "Unintimidated," named for Walker's book about surviving his 2012 recall in Wisconsin.

Some non-partisan watchdog groups argue that by waiting so long to put up what is known in political circles as "the wall," they are following the letter of the law, but not the spirit.

"It sure doesn't hurt being a non-candidate candidate who leads his own super PAC until which time he hands it over to be run by his most intimate political supporters upon officially announcing a presidential run," said Dave Levinthal of the Center for Public Integrity. "The unspoken message will still be crystal clear: Support me with money and support me with lots of it."

Advisers to both Walker and Bush insist they go beyond the letter of the law and limit communication internally in order to avoid questions when they officially declare for president.

But that is being met with skepticism.

"It's basically the difference between a baseball manager sending in hand signals to his players and directly whispering the message into their ears," Levinthal said. "The message more or less gets through all the same."

Even after that so-called wall goes up between the campaign and super PAC, presidential candidates are allowed to attend super PAC fundraisers; they just can't explicitly ask for money.

Political operatives of all stripes say that makes a big difference, because it's harder for donors to say no to a candidate and because donors are usually wealthy, successful people who like their egos stroked.

It's not just money

Bush advisers insist it's not just the desire to raise tens of millions of dollars early that is driving his decision to wait until at least June to officially declare his candidacy. They note that he left public office nearly 10 years ago, so unlike sitting governors and senators who are running for president, he had no existing infrastructure to support him either on policy, politics or logistics. Bush sources are quick to remind that he is building an organization from the ground up, which takes time.

Sitting Govs. Walker and Christie, who have infrastructure around them, also still have day jobs with a very important role -- guiding, and eventually signing, their state budgets.

In both Wisconsin and New Jersey, the budgets must be signed by the end of June, and aides to both Walker and Christie call it critical not to declare their candidacies for president before finishing the budget back home.

That's especially true for Walker, who became a national GOP figure in large part because of his 2011 budget fight relating to Wisconsin unions.