MANCHESTER, N.H. >> President Barack Obama’s health care overhaul is coming under renewed attack as some of the nation’s leading conservatives gather in New Hampshire for a summit that some consider the unofficial kickoff for the state’s 2016 presidential selection process.

Several potential Republican White House contenders — among them Kentucky Sen. Rand Paul, Texas Sen. Ted Cruz, and former Arkansas Gov. Mike Huckabee — headline a conference Saturday in Manchester, N.H., hosted by the conservative groups Citizens United and Americans for Prosperity.

Scheduled speakers also include real estate mogul Donald Trump, former House Speaker Newt Gingrich, Utah Sen. Mike Lee and New Hampshire Sen. Kelly Ayotte. The gathering highlights the role of Koch Industries, the giant conglomerate headed by the billionaire brothers Charles and David Koch.

The Koch-affiliated Americans for Prosperity has already spent millions of dollars on health care-related attack ads aimed at Democratic senators in New Hampshire, North Carolina, Alaska, Colorado, Iowa and elsewhere. That’s made the Koch brothers a prime target for Democratic criticism.

The summit comes as prospective presidential candidates begin to step up appearances in key states ahead of the 2016 presidential contest, even though New Hampshire’s first-in-the-nation presidential primary isn’t planned for another two years.

“It’s the unofficial kickoff of the 2016 process,” said Republican operative Mike Biundo, who managed Rick Santorum’s last presidential campaign.

The early speakers bashed the Democratic-backed health care overhaul, a central issue in the GOP’s midterm election strategy despite reports of strong enrollment figures. Some noted this week’s resignation of Health and Human Services Secretary Kathleen Sebelius, who presided over the law’s implementation.

Ayotte asked, “What took so long?”

“I have heard from the people of New Hampshire, and the people of New Hampshire want us to work to repeal this law and replace it with common-sense reforms,” she said.

Sebelius resigned on Friday, days after the Obama administration announced that enrollment in the Affordable Care Act had grown to 7.5 million Americans, a figure that exceeded expectations and gave Democrats a surprise success after a disastrous rollout. It was welcome news for Democrats who’ve been forced to defend their support for the unpopular law derided by critics as “Obamacare.”

As potential presidential candidates jockey for position, the stakes are high for the November midterm elections, where Republicans are fighting to claim the Senate majority. The shift could block Obama’s legislative priorities in the final two years of his presidency.

In a conference call Friday, Rep. Chris Van Hollen, D-Md., insisted that “Democrats are not running away from the Affordable Care Act.”

Democratic National Committee spokesman Mike Czin noted that Republican opposition to the health care law was the foundation of the GOP’s unsuccessful political strategy in 2012. He said that the debate has changed now that the law has been implemented and millions of people are enjoying its benefits.

“That’s a debate that we’re going to have, and we’re eager to have,” Czin said.

At the same time, Van Hollen, who leads House Democrats’ campaign efforts, called for Republicans to defend their support for a GOP budget plan introduced this week that would repeal the health care law, transform Medicare, reintroduce the “doughnut hole” for prescription drug costs and enact deep cuts in education.

“Republicans in Iowa and New Hampshire are supporting an agenda that hurts middle-class families, hurts women and will benefit billionaires like the Koch brothers,” Van Hollen said.