The cornerstone of ObamaCare, the new insurance marketplaces at the heart of the debate over a government shutdown, opens for business Tuesday.

The rollout of America’s new healthcare system is a pivotal moment for both President Obama and his Republican critics, and that’s a core reason why it has brought the federal government to a grinding halt.

{mosads}Republicans have fought fiercely to delay, defund or partially repeal the healthcare reform law in exchange for funding the rest of the federal government — proposals Obama and Senate Democrats have flatly rejected.

Obama said Monday, “The Affordable Care Act is moving forward. That funding is already in place. You can’t shut it down.”

No agreement on the spending bill had been reached as of The Hill’s press time, increasing the likelihood that the first day to sign up for ObamaCare would also be the first day of a government shutdown caused largely by the partisan divide over ObamaCare.

“Shutdown or no shutdown, we’re ready to go,” Health and Human Services (HHS) Secretary Kathleen Sebelius told reporters Monday.

The new marketplaces, also known as exchanges, officially open at 8 a.m. in most states. After years of political back-and-forth about the law’s effect on costs, Tuesday will be consumers’ first chance to compare their options in their state, based on their specific circumstances — assuming the exchanges work as intended.

Obama and administration officials have warned repeatedly that some implementation snafus are inevitable as the exchanges launch.

But Republicans will surely trumpet any reported snags in the initial rollout.

“We know that there are opponents who will be desperate for anything to go wrong,” Sebelius said Monday.

For Obama and Democrats, the danger of a rocky rollout is obvious: It would help validate Republican criticisms of the law just as the GOP is insisting on chipping away at the law as part of any bill to fund the federal government.

If the healthcare rollout goes relatively smoothly, however, Republicans could find themselves in the hot seat.

A botched rollout could also have real-world implications for the law’s success, if problems are big enough or last long enough to make consumers think twice about even exploring their options through the exchanges.

Democrats have been careful to avoid setting almost any expectations for the initial rollout.

Sebelius and other HHS officials sidestepped questions Monday about the specific metrics they will be tracking when the exchanges launch, and would not say when “glitches” would cross the line into serious problems.

“In terms of success at the end of the day tomorrow — our systems are online; people are able to access them; people are coming to our call center; they are actively seeking educational content and information, and taking the initial steps in a process of education and eventual enrollment,” one HHS official said.

Judging the healthcare law based solely on how Oct. 1 goes would be impossible, however. As officials have noted repeatedly, Tuesday is the first day of an enrollment window that lasts six months.

The administration says that would be plenty of time to iron out any technical problems, and it has always planned for its public relations campaign to span the entire six-month enrollment window.

HHS has set aside roughly $10 million for television ads encouraging people to enroll in new coverage options through the exchanges, and Sebelius said Monday that celebrities would continue to help promote the law.

“Oct. 1 is not the end of anything; it is the beginning,” Sebelius said.

The administration is hoping to enroll about 7 million people through the exchanges over the next six months, including roughly 2.5 million young adults. Getting young, healthy people into the system is necessary to keep premiums from skyrocketing.

Although HHS officials declined to set expectations for the exchanges, Obama’s political allies are apparently confident that the marketplaces will work.

Democrats and their allies have scheduled several events across the country Tuesday to highlight the exchanges, including Capitol Hill events featuring Democratic congressional leaders.

The law’s opponents will surely be monitoring reports of any glitches, but some leading critics aren’t planning a big counteroffensive to Democratic events.

The conservative advocacy group FreedomWorks, which has spearheaded the campaign urging people to burn fictitious “ObamaCare draft cards,” said it doesn’t have any anti-ObamaCare events scheduled for Tuesday.

— This story was updated at 8:15 p.m.