A quirky primary rule propelled Trump to victory in 2016, but it could come back to bite him in 2024

• Trump is railing against Nikki Haley for trying to get non-traditional Republicans to vote in New Hampshire.
• He once used open primaries to squash Ted Cruz in 2016.

Former President Donald Trump might owe Sen. Ted Cruz an apology.

And no, it's not about the long list of things Trump has said about Cruz and his family. Rather, it's about how Cruz and his allies complained in 2016 about how non-Republican voters helped hand the nomination to Trump. They even tried to formally change the party's rules to incentivize states to move away from elections that allowed independent and possibly even Democratic voters to weigh in on the Republican presidential race.

Trump ran away with the 2016 Republican presidential nomination in part due to his sheer dominance in these so-called "open primaries." While at the same time, Cruz struggled to keep up by winning in closed primaries and caucuses.

Now, Trump is sounding the alarm at the possibility that former UN Ambassador Nikki Haley could use his old playbook against him.

"As you know, Nikki Haley, in particular, is counting on the Democrats and liberals to infiltrate your Republican primary, you know that, that's what's happening," Trump said at a rally in New Hampshire on Tuesday evening. "A group of people coming in that are not Republicans and it's artificially boosting her numbers here, although we are still leading her by a lot."

It's worth noting that there is nothing illegal about Haley's strategy. New Hampshire law has long allowed unaffiliated voters to vote in the presidential primary. At issue is a long-running discussion that far pre-dates Trump about who should pick either of the major party's presidential nominees. Long ago, party bosses held sway but the progressive era saw a push for broader primary elections.

While some Republicans detest more open primaries, such rules challenge candidates to start building the broader coalition they would need to win the White House in a general election. It's a fact Haley's campaign is quick to bring up.

"Nikki has always believed that the Republican Party has to be about addition, not subtraction. Republicans have lost the popular vote in the last seven of the eight elections," Haley spokesperson AnnMarie Graham-Barnes said in a statement to Insider. "We lost races we should have won in 2018, 2020, and 2022. If Republicans want to start winning again, we have to start bringing in people fed up with Joe Biden's disastrous administration, not pushing people away."

Most states operate at least a somewhat open primary. According to the National Conference of State Legislatures, only eight states have completely closed presidential primaries — most notably the delegate-rich state of Florida.

Trump once ran up the score in open primaries.

Trump's advantage in open primaries in 2016 was unmistakable.

A CNBC tally from March 2016 found that the future president won a greater percentage of primaries and caucuses in states with open contests. There's not a perfect explanation as to why though. Entrance and exit polling at the same didn't show independents giving Trump a major advantage — such surveys can be unreliable though. The split-screen between the two types of contests began almost immediately. Despite holding a polling advantage in the weeks leading up to the 2016 Iowa caucuses, Trump narrowly lost the state to Cruz. Iowa, unlike New Hampshire, is only partially open since the state requires voters to register with a party to caucus. Voters can change their affiliation on caucus night.

Trump's rout also stuck with Cruz's allies. Pro-Cruz delegates came to the 2016 Republican National Convention determined to change the party's rules by incentivizing states to move away from open primaries or doing away with the practice altogether. Their efforts never amounted to any major changes.

The former president's campaign did not respond to Business Insider's request for comment. But his apparent reversal on open primaries is quite simply yet another sign of his hostile takeover of the GOP. Trump is no longer an outsider now. And as such, he sees little need for such voters.