Outside groups are scrambling to figure out how to fill the $90 million in ad funding for Obamacare that was cut by the Trump administration.

Advocacy groups said they were surprised and shocked by the Trump administration's abrupt decision last month to provide only $10 million for outreach for Obamacare's 2018 open enrollment. That figure is 90 percent below the $100 million the Obama administration shelled out for 2017.

The cut was coupled with a nearly 40 percent decline in grants to navigators, which are nonprofits tasked with helping people sign up for Obamacare. Navigators are expected to get $36.8 million for 2018, down from $62.5 million this year.

Obamacare advocacy groups say they are meeting to figure out how to make up the gap. But some say additional funds won't be enough because the Trump administration, which oversees healthcare.gov, knows best where to put the money.

"It is impossible to replace the funding and it is impossible to replace the system because the administration knows who is coming to visit the website and might need a nudge," said Leslie Dach, campaign chairman for the pro-Obamacare advocacy group Protect Our Care, during a call with reporters last week.

Dach said the administration knows who enrolled for this year and plays a key role in communication. "They hold the keys to communication, and they made it clear they will keep those and not do anything to make the law work and actively undermine it," he said.

Trump critics have said the administration is working to sabotage Obamacare. They point to the many statements Trump has made saying he wants to let the law implode.

Officials with Health and Human Services have said the smaller amount of money will be spent more wisely. They point out that Obamacare is entering its fifth enrollment period and awareness is greater now, thus negating the need for more ad dollars.

Other groups said they face a major obstacle to filling the gap since open enrollment starts on Nov. 1.

"Regardless of all the nonprofit organizations that decide to get involved, there is still a significant gap than what we have had in the past," said Elizabeth Hagan, associate director of coverage initiatives for Families USA. "To be able to fill that in within the next two months is a tall order."

The federal government is required under the Affordable Care Act to perform outreach and operate call centers. The law doesn't specify how much must be spent on advertising, said Timothy Jost, a law professor at Washington & Lee University in a blog post for the journal Health Affairs.

User fees paid by Obamacare insurers provide funding for ads and outreach. However, HHS officials have said the Obama administration put in more money beyond the user fees, but has not specified how much.

HHS has said it plans to put some of the money from outreach to clamp down on fraudulent Obamacare signups and third parties to create websites that complement healthcare.gov, which residents in 39 states and the District of Columbia use to buy Obamacare plans.

Some organizations were already planning to boost their ad spending after the Trump administration cut Obamacare ad funding by $5 million at the tail end of the 2017 open enrollment.

California's state-run Obamacare exchange, one of 12 in the country, boosted its marketing and outreach budget by $5 million to a total of $111 million.

"The additional funding will be used to increase the number of television and radio ads around key dates throughout the upcoming open-enrollment period, which will run from Nov. 1 through Jan. 31 in California," said James Scullary, a spokesman for Covered California, the state's exchange.

The federal open enrollment also starts on Nov. 1 but ends on Dec. 15.

Covered California plans on submitting a report this week on how investing in marketing and outreach can boost enrollment.

Democrats are also trying to add outreach funding to a bipartisan stabilization deal on Obamacare's marketplaces, according to a report in The Hill. They are likely to meet stiff resistance from Republicans who want a narrower package or want to take another shot at Obamacare repeal.