New Jersey Gov. Chris Christie at an anti-human trafficking forum in Phoenix on Friday. (Ross D. Franklin/AP)

When Gov. Chris Christie (R) decided that New Jersey would expand Medicaid under the Affordable Care Act , he opened the door to health-care coverage for 104,000 of the poorest Garden State residents, a move he said would save the state $227 million this year alone.

But he also may have complicated his hopes of making it through a competitive Republican presidential primary in 2016, when rivals will be looking for any opportunity to distinguish themselves from the rest of the crowded field.

Those rivals won’t forget that Christie expanded Medicaid — a move they will paint as an unmistakable embrace of Obamacare, as the health-care law is commonly known.

Christie is one of eight Republican governors to accept the expansion, including at least one other potential presidential candidate, Ohio Gov. John Kasich . The jockeying over Medicaid is a hot topic this week in Phoenix at the Republican Governors Association’s annual meeting, and it is shaping up as one of the earliest fights in the shadow campaign for the Republican nomination.

“On the case of the New Jersey governor, I think embracing Obamacare [and] expanding Medicaid in his state is very expensive and not fiscally conservative,” Sen. Rand Paul (R-Ky.) said this week on CNN, adding: “That fact, I would say, would lead towards you making the conclusion that it’s not a very conservative proposal.”

Asked by a reporter in Phoenix whether accepting a Medicaid expansion would make securing the Republican nomination more difficult, Kasich said: “Is that how you’re going to make a decision?

“Anybody who would be making the decision from that standpoint I wouldn’t want to be supporting for president,” Kasich said. “I think all things kind of fade over time.”

Expanding Medicaid would make most adults who earn up to 138 percent of the federal poverty limit eligible for coverage in that state. The federal government will cover 100 percent of the costs of the expansion for the first few years of the program, and 90 percent of the costs after that.

Most Republican governors tout the fact that they turned down a Medicaid expansion, saying they don’t trust the federal government to keep its commitment to cover 90 percent of the costs in the future. But many also are loathe to criticize their colleagues who decided differently.

Indiana Gov. Mike Pence (R), who did not accept an expansion, said in an interview that it “was a decision that every governor had to make based upon the unique circumstances in their own state.”

Gov. Nikki Haley of South Carolina, home to the first-in-the-South presidential primary, said, “I think it’s going to be an issue, certainly, that is talked about just like others. But it won’t be the sole issue.”

But several political strategists and analysts said the negative television advertisements against Christie and Kasich on Medicaid virtually write themselves.

“In the standard attack ad, the governor’s face will turn into President Obama’s, and the voice-over will say that a vote for the governor is a vote for Obamacare,” said John Pitney, a political scientist at Clare­mont McKenna College and a former top staffer at the Republican National Committee. “These spots will feature more morphing than a Terminator movie.”

One strategist working for a potential 2016 candidate, who spoke on the condition of anony­mity while discussing internal strategy so far in advance of the GOP primaries, pointed to George Voinovich, the former Republican governor and senator from Ohio. Voinovich, then governor, was on top of then-Sen. Bob Dole’s list of potential vice presidential running mates in 1996 until a Wall Street Journal editorial pointed out his record of raising taxes.

The same strategist said that, after Kasich moved to expand Medicaid, he crossed Kasich’s name off the list of his boss’s potential rivals.

Christie formally took the reins at the RGA after a governors-only meeting Thursday. The new position will give Christie an opportunity the hobnob with high-dollar donors and travel around the country laying the groundwork for a possible presidential bid, as former RGA presidents Mitt Romney and Rick Perry did before him.

Christie succeeds Louisiana Gov. Bobby Jindal, another Republican with national ambitions — and one of the majority of Republican governors who said no to a Medicaid expansion.

“I do think everybody should have health insurance,” Jindal said in an interview. “I just think ACA and Medicaid expansion were the wrong way to solve that problem.”

But Jindal declined to draw a contrast with Christie. “Every governor has to make the best decision for their state, so I’ve never second-guessed or criticized or questioned those governors who made a different decision than I did,” he said. “Let’s stop thinking about 2016.”

Iowa Gov. Terry Branstad (R), who signed a Medicaid expansion into law this year and has made clear that he wants a governor to win his party’s presidential nomination, said voters in his caucus state would look beyond Medicaid when evaluating candidates.

“I think people are going to look at the big picture,” Branstad said. “Medicaid is just one small issue.”

Dan Balz contributed to this report.