The most robust group working to stop Donald Trump from winning the Republican presidential nomination, Our Principles PAC, is shifting its focus from stopping Trump at the polls to limiting his success in acquiring delegates, a potential major weakness for the Republican front-runner.

The group is using complicated Republican Party procedure to outmaneuver Trump in the tedious delegate selection process necessary to win the party's nomination.

“Moving beyond Wisconsin, we’re not focusing on statewide wins as much as the delegate mass,” co-founder of Our Principles PAC, Katie Packer, told NBC News. “We’re going to be working very aggressively to elect delegates that aren’t for Trump.”

The super PAC has been using its resources to persuade voters against Trump through television, digital and mailed advertising. But as the delegate selection process becomes critical in the three-person race for the nomination, the group is using the very complicated and multi-layered process to beat Trump.

As state Republican parties hold their delegate selection committees over the next two months, Our Principles PAC is deploying people and resources to ensure that delegates selected to nominate the party's standard bearer at the GOP convention this summer in Cleveland are not Trump supporters.

“It’s an Achilles Heel for him,” Packer said, referring to Trump’s inability to secure supportive delegates. “They just don’t have the organization you have to have to be the nominee.”

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For instance, Colorado, which didn’t hold a primary, has been holding its delegate selection meetings in each of its counties over the past two weeks. The group has been “communicating with” party activists there to make sure they know who the non-Trump delegates are. Of the six delegates selected so far, Packer said, none are Trump supporters. The state’s delegate selection process should be complete Sunday.

Michigan is another state they will be watching closely. It holds its party convention to elect delegates next week.

It’s a tactic that proved successful already. The group worked diligently at North Dakota’s state convention last weekend. In addition to anti-Trump mobile digital ads targeting the convention’s attendees, they called all registered participants to sway them away from Trump.

Combining their efforts with the organized campaign of Sen. Ted Cruz, it worked. Of the 25 delegates elected to represent North Dakota at the Republican National Convention this summer, Trump came away with no committed delegates.

While only a fraction of states have awarded delegates, Trump has been out-maneuvered. It’s a complicated process where Republican Party activists in each state gather to elect delegates for the Republican Party’s national presidential nominating convention in July. According to officials at the Republican National Committee, more than 90 percent of delegates are bound to the winner of the state's primary or caucus. But if no candidate achieves a majority – 1,237 delegates – then balloting at the national convention goes to a second round where most of those delegates become unbound - they are free to support whomever they want. So a bound Trump delegate can support Cruz or any other person in a second round, possibly making this delegation selection process critical.

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Trump has called the system “corrupt,” but an outsider who has few political veterans working for him, he has been disorganized and undisciplined on the critical delegate selection process. He is trying to catch up by hiring Republican strategist Paul Manafort to run his state convention operation.

Packer, meanwhile, is a presidential campaign veteran with experience at state conventions. She was deputy campaign manager for Mitt Romney’s 2012 presidential run and even though he secured the nomination in April through the voting process, he took no chances and worked the state conventions to ensure that his supporters won delegate slots.

Our Principles PAC's focus on state conventions isn’t a complete replacement for their focus on primary voters. They will simultaneously work to ensure that Trump doesn't obtain 1,237 delegates, because if he does then it would make the entire focus on the states' delegate contests mute.

Packer believes that if Trump loses Wisconsin, which is voting Tuesday, there is no way that he can obtain enough delegates through the voting process, making a second round of balloting at the national convention necessary.

“By our count if he loses Wisconsin, he does not get to 1,237, period,” Packer said, making Wisconsin a critical juncture in the campaign.

They have been “literally saturating” talk radio with anti-Trump ads in the state. They’ve aired issue-specific television in ads in smaller markets, including one on Trump’s position on eminent domain. They also have been targeting voter lists of people who supported Gov. Scott Walker and 2012 Republican nominee Mitt Romney.

The group has spent more than $1.2 million in Wisconsin on television advertising, according to advertising tracking group SMG Delta and NBC News, bringing their primary spending to more than $10 million, according to SMG Delta and filings to the Federal Election Commission.