Washington (CNN) As a presidential candidate, Donald Trump repeatedly and forcefully promised to expand access to drug treatment, strengthen prevention options and address the scourge of drug addiction after hearing about many Americans' struggle with opiate abuse.

Now, Trump's rhetoric as a candidate is running headfirst into the realities of Trump the president as Republicans promote their plan to repeal and replace the Affordable Care Act.

The current version of the Trump-backed Republican health care plan would end the Obamacare requirement that addiction services and mental health treatment be covered under Medicaid in the 31 states that expanded the health care program. The GOP plan would instead leave up to states -- and their budgets -- to decide whether to cover drug treatment and mental health services under Medicaid. That's a decision advocates say could put the most vulnerable opiate abusers in greater risk, thanks to near-constant pressure on state budgets.

"If you cut off even the essential funding for people to get that treatment, they simply won't get it," said Dr. David C. Lewis, the founder of the Brown University Center for Alcohol and Addiction Studies. "That is the problem."

Dr. Andrew Kolodny, the director of the Opioid Policy Research Collaborative at Brandeis University, called the proposed change a "step in the wrong direction at a time when America's most urgent public health crisis is an addictive disorder."

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Trump: We will stop drugs from pouring into US

Medicaid expansion provides drug treatment for close to 1.3 million people, according to a study by Harvard University and New York University . Drug treatment advocates and a bipartisan mix of lawmakers, some of whom were heartened by Trump's rhetoric about drug treatment during the campaign, are concerned that the repeal and replace effort now being marshaled by the White House will dramatically alter drug treatment programs at a critical time in the fight against opioid addiction.

Despite this, Trump has made it clear to House and Senate Republicans that Medicaid expansion is on the table in negotiations over repealing the health care law.

After conservative Republicans rejected a part of the plan that would have ended Medicaid expansion funds authorized under Obamacare by 2020, multiple White House sources told CNN that Trump and his staff have privately begun lining up behind calls to roll back its funding earlier than expected.

"It provides states with flexibility over how Medicaid dollars are spent, giving power from Washington and back to local government," Trump said Friday.

But some health care experts say that is exactly the problem.

Richard G. Frank, a professor of Health Economics at Harvard University who authored a study on Medicaid and drug treatment, said Trump's actions will likely mean less funding for substance abuse.

"The states then have a choice: They can continue to take on those responsibilities and pay for it out of their own budgets, or, if they are under pressure, they have to scale back," Frank said. "Historically, states have been loath to cover substance abuse treatment."

The decision to go ahead with the Medicaid changes will also likely anger Republican governors who accepted federal funds for the program's expansion, in part, to combat drug abuse.

"Thank God we expanded Medicaid because that Medicaid money is helping to rehab people," Republican Ohio Gov. John Kasich said in January.

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Opioid abuse played an outsized role throughout the 2016 campaign, with both Hillary Clinton and Trump speaking at length about the ever-growing crisis in states like New Hampshire, West Virginia and Kentucky.

Opioid addiction and overdose have reached epidemic levels, according to the Center for Disease Control . The agency released a study last month that found 25% of all drug overdoses were related to heroin in 2015. Only 6% of all overdoses were related to the drug in 1999.

In one of his first trips to New Hampshire, a state that has been plagued by opioid abuse, Trump spoke at length about combating the public health problem.

"First, we have to support locally based and locally run clinics," Trump said in 2015. "In the meantime, people are getting hooked, and we're going to take care of those people."

Almost a year later, Trump was still talking about opioid abuse and promised to "end Medicaid policies that obstruct inpatient treatment."

"Finally, we will give people struggling with addiction access to the help they need," Trump said in October 2016, weeks before Election Day. "I would dramatically expand access to treatment slots."

Trump even made these promises as president. During his joint address to Congress in February, Trump promised to "expand treatment for those who have become so badly addicted."

With Republicans now considering a plan that would cut into those promises, four Republican senators -- Rob Portman of Ohio, Shelley Moore Capito of West Virginia, Cory Gardner of Colorado and Lisa Murkowski of Alaska -- sent a letter to Senate Majority Leader Mitch McConnell to stress the need to protect Medicaid's role in providing drug treatment services.

Photos: President Trump's first address to Congress US President Donald Trump addresses a joint session of Congress for the first time on Tuesday, February 28. Behind him, from left, are Vice President Mike Pence and House Speaker Paul Ryan. Hide Caption 1 of 22 Photos: President Trump's first address to Congress Trump speaks at the beginning of his address. Hide Caption 2 of 22 Photos: President Trump's first address to Congress A wide view of the House chamber. Hide Caption 3 of 22 Photos: President Trump's first address to Congress Supreme Court justices watch the speech. From left, in front, are John Roberts, Anthony Kennedy, Stephen Breyer, Sonia Sotomayor and Elena Kagan. Hide Caption 4 of 22 Photos: President Trump's first address to Congress Maureen Scalia, the widow of late Supreme Court Justice Antonin Scalia, is applauded during the speech. Hide Caption 5 of 22 Photos: President Trump's first address to Congress Carryn Owens, center, cries as she is applauded by the chamber during Trump's speech. Owens' husband, Navy SEAL William "Ryan" Owens, recently was killed during a mission in Yemen. "Ryan died as he lived: a warrior and a hero, battling against terrorism and securing our nation," Trump said. The applause lasted over a minute, which Trump said must be a record. Hide Caption 6 of 22 Photos: President Trump's first address to Congress Trump also recognized Megan Crowley, a college student who, at 15 months old, was diagnosed with Pompe disease and wasn't expected to live past age 5. Her father founded a pharmaceutical company to find a cure. "Megan's story is about the unbounded power of a father's love for a daughter," Trump said. "But our slow and burdensome approval process at the Food and Drug Administration keeps too many advances, like the one that saved Megan's life, from reaching those in need." Hide Caption 7 of 22 Photos: President Trump's first address to Congress Muslim activist Fauzia Rizvi, a guest of US Rep. Mark Takano, watches Trump's address. Hide Caption 8 of 22 Photos: President Trump's first address to Congress Members of Trump's Cabinet applaud the President. From left are Defense Secretary James Mattis, Education Secretary Betsy DeVos, Attorney General Jeff Sessions, Treasury Secretary Steve Mnuchin and Secretary of State Rex Tillerson. Hide Caption 9 of 22 Photos: President Trump's first address to Congress Trump smiles during his speech, which lasted over an hour. Hide Caption 10 of 22 Photos: President Trump's first address to Congress House Minority Leader Nancy Pelosi listens to the speech. Hide Caption 11 of 22 Photos: President Trump's first address to Congress The President waves before starting his speech. Hide Caption 12 of 22 Photos: President Trump's first address to Congress Trump is applauded after arriving in the House chamber. Hide Caption 13 of 22 Photos: President Trump's first address to Congress Trump shakes hands with Ryan before starting. Hide Caption 14 of 22 Photos: President Trump's first address to Congress Trump shakes hands on his way through the chamber. Facing the President here are Health and Human Services Secretary Tom Price, left, and UN Ambassador Nikki Haley. Hide Caption 15 of 22 Photos: President Trump's first address to Congress Trump stands in the doorway of the House chamber while being introduced. Hide Caption 16 of 22 Photos: President Trump's first address to Congress Pence confers with Ryan before Trump's speech. Hide Caption 17 of 22 Photos: President Trump's first address to Congress First lady Melania Trump, bottom right, is applauded as she arrives in the chamber. Hide Caption 18 of 22 Photos: President Trump's first address to Congress US Sen. John McCain, left, talks with US Sen. Lindsey Graham before Trump arrived. Hide Caption 19 of 22 Photos: President Trump's first address to Congress US Sens. Cory Booker and Bob Menendez arrive in the House chamber. Hide Caption 20 of 22 Photos: President Trump's first address to Congress Many Democrats wore white as a nod to the women's suffrage movement. Posing for a photo here are, from left, US Reps. Brenda Lawrence, Joyce Beatty, Marcia Fudge and Val Demings. Hide Caption 21 of 22 Photos: President Trump's first address to Congress US Sen. Bernie Sanders arrives for the speech. Hide Caption 22 of 22

"As the largest payer of mental health and substance use services in the United States, it is critical that any health care replacement provide states with a stable transition period and the opportunity to gradually phase in their populations to any new Medicaid financing structure," the letter said.

The senators asked that the health care plan "provide stability and certainty" for those who rely on Medicaid for drug treatment services.

Multiple White House spokespeople did not respond to questions about whether Trump is still committed to his drug treatment pledges and whether he specifically backs the plan to alter the way Medicaid covers drug treatment.

But many advocates are concerned with the possible changes because Medicaid has been found to be the most effective way to combat opioid abuse -- and this is a critical time in the fight.

"States are grappling day-to-day with the vast and deadly public health crisis," a study by the Robert Wood Johnson Foundation found in 2016 . "Medicaid is the most powerful vehicle available to states to fund coverage of prevention and treatment for their residents at risk for or actively battling opioid addiction."

Chuck Ingoglia, the senior vice president of Public Policy at the National Council for Behavioral Health, said these changes come at a critically serious time for the fight against addiction and mental health.

"We are really worried that this loss of protection and the financial pressure will force states to scale back," Ingoglia said. "Meaning there will be a whole bunch of substance abuse benefits that we think are going to be taken away."

Ingoglia said a member of his family, who deals with mental health issues, has benefited from the Medicaid expansion and the possibility of that going away is troubling for him.

"I really worry what this is going to mean for her," he said. "I really worry."