WASHINGTON (Reuters) - President Barack Obama on Thursday urged more than 25,000 volunteers and advocates who dialed in to a White House conference call to pull out the stops to boost the number of people signing up for Obamacare health insurance plans.

U.S. President Barack Obama speaks at a rally to support Hillary Clinton's campaign in Las Vegas, Nevada October 23, 2016. REUTERS/Kevin Lamarque

Obama warned it will be challenging to overcome the skepticism about the plans given an onslaught of headlines about surging premium prices, but he said the stakes are high.

“I think we’re at a critical time where we have to show that this program works for people, if they just see what their options are,” he told the volunteers, who work in their communities to encourage and assist enrollment.

Americans who do not receive health insurance through their employer or through Medicare or Medicaid programs shop online for subsidized insurance plans starting Nov. 1 until the end of January.

The average premium for benchmark 2017 plans sold on healthcare.gov rose 25 percent compared with 2016.

Obama said tax credits will help more than seven in 10 shoppers get a plan for less than $75 per month, but said many may not bother looking because they have heard about spiking costs.

“We’re going to have to clear the bugs off the windshield so people can see the road ahead, and that’s where you guys come in,” Obama said.

The law has been fought by Republicans in Congress, who said it creates unwarranted government intervention in personal healthcare and private industry.

Several big insurers, including UnitedHealth Group Inc UNH.N, Aetna Inc AET.N and Humana Inc HUM.N, are pulling out of the online marketplaces selling the subsidized plans, citing bigger-than-expected financial losses.

Aetna Chief Executive Mark Bertolini said on Thursday that the earliest his company may return to the marketplaces would be 2019.

Obama said more young and healthy people need to sign up for plans. That would offset insurers’ costs of covering members with serious illnesses.

Obama has said there are a series of improvements that could be made to his signature domestic policy achievement - the 2010 Affordable Care Act - if Congress and the next president, who will take office on Jan. 20, can work together.

“Part of what we can do this time is to overcome the skeptics, to prove people wrong, and to provide momentum so that when the next administration comes in, they are starting from a position of strength,” Obama told the conference call.