Barack Obama declared on Thursday that 8 million Americans have signed up for health insurance under the Affordable Care Act, and called on Republicans to abandon what he described as “endless, fruitless” attempts to unravel his signature health reforms.

Congressional budget forecasters had predicted that about 7 million people would enrol through the act's healthcare exchanges. The administration surpassed that benchmark despite disastrous technical problems that afflicted the central healthcare website, rendering the insurance exchanges practically unusable at its launch in October last year.

For the third time in as many weeks, Obama sought to seize on the better-than-expected enrolment figures to declare that his flagship health law had defied its critics.

“They still can’t bring themselves to admit the Affordable Care Act is working,” Obama said of his Republican opponents. “They said nobody would sign up; they were wrong about that. They said it would be unaffordable for the country; they were wrong about that.”

House Republicans have held more than 50 symbolic votes to repeal the ACA, widely known as Obamacare, passed almost four years ago.

GOP strategists have have already made Obamacare the focus of two election campaigns, in 2010 and 2012, and are planning to campaign heavily on the issue in the run-up to November’s midterm elections.

Political analysts are divided over the impact the ACA will have on the Republican campaign to take control of the Senate. However, Democrats hope the unexpected turnaround in enrolment figures will undermine Republican attempts to focus on an issue many voters say they are growing tired of.

Obama used Thursday’s announcement at the White House to implore Republicans to move on. “I know that every American isn’t going to agree with this law," he said. "I think we can agree it is well past time to move on as a country, and refocus our energies on the issues the American people are most concerned about.”

It is the third time Obama has made such arguments at this month. He made a similar case during an upbeat speech after the formal enrolment period closed on 31 March, when about 7.1 million people had signed up. He repeatedly urged the country to move on last week, when announcing the departure of his health secretary, Kathleen Sebelius.

The extra 900,000 enrolees over the past two weeks were the result of a decision by Sebelius to extend the enrolment period for people who had trouble signing up because of the failing website.

The glitches took months to repair, slowing the initial pace of enrolment for affordable healthcare coverage, and Obama conceded he and Sebelius had been “bruised” by the debacle.

The so-called “grace period” provided some Americans with a final, two-week period to sign up for a new insurance plan, a window that closed earlier this week.

A crucial determinant of the ACA’s success will be the extent to which young and healthy people enrol for new healthcare plans. Obama said 35% of those who signed up belonged to the crucial 35-and-under demographic.

The White House later said that 28% are between 18 and 34, figures that were “virtually the same youth percentage” that signed up under the Massachusetts healthcare system upon which Obamacare is loosely modelled.

Republicans say the top-line figures for enrolment announced by the administration in recent days are misleading, because the administration has not said how many of those who have signed up for insurance closed the deal by paying their first month’s premiums.

Neither has the Obama administration yet disclosed how many people lost their plans because they did not meet the requirements of the ACA, contrary to a promise from the president that anyone who wanted to keep their premium could do so.

Obama did not provide more clarity on either front on Thursday, but he did tacitly acknowledge problems with the rollout. “There are people who have seen problems with the law,” he said. “There may have been people affected in ways they were not happy about. That is a far smaller number than the millions of people who have signed up. That doesn’t mean we shouldn’t be concerned about it.”

The healthcare exchanges are one central plank of the ACA, a monumental piece of legislation designed to significantly expand healthcare coverage. Over the next decade, it is predicted that 25 million Americans will receive healthcare coverage as a result of the changes.

The law has overhauled the health insurance market, creating online healthcare exchanges and an individual mandate requiring almost all Americans to obtain coverage or pay a penalty.

In many states it also significantly expanded Medicaid, which provides healthcare to low-income Americans.

However, 25 states have decided not to accept federal funding to expand Medicaid, a move Obama said was ideologically driven. “This shouldn't be a political football,” he said of his healthcare law. “This should be something that we take for granted; in this country, you should be able to get affordable healthcare, regardless of how wealthy you are.”