The Republican-controlled Senate early Saturday advanced a tax bill that would topple a controversial provision in Obamacare after repeated failed efforts earlier this year to do so through healthcare legislation.

The move represents progress in the GOP's eight-year-plus effort to chip away Obamacare since Democrats passed it along party lines. The individual mandate, the portion slated for repeal, is one of the most unpopular portions of Obamacare. Proponents insist the mandate is key to making the law work, but opponents either dismiss it as ineffective or slam as an example of federal overreach.

The Senate-passed bill, the Tax Cuts and Jobs Act, contains a provision that would repeal the individual mandate penalties in Obamacare that obligate people have health insurance or otherwise pay a fine. Its inclusion in the tax bill was spurred by Sen. Tom Cotton, R-Ark., with backing from the president, despite initial caution from some Republicans who were concerned that adding healthcare to the bill would sink their efforts.

The final version of the tax bill must be worked out in conference with the House and passed again by both chambers before going before the president. The House version of the tax bill does not contain the repeal of the Obamacare penalty, but President Trump supports it and earlier this year the House passed a healthcare bill that repealed it.

Left more uncertain is the trade-off Republican leaders made in order to secure support from all but one Senate Republican. Sen. Susan Collins, R-Maine, who voted against GOP efforts to repeal parts of Obamacare this summer, came onboard for the tax plan after promises from leadership that the Senate would bring a bipartisan bill to the floor known as Alexander-Murray. Collins also was able to secure a pledge to bring a two-year, $5 billion-a-year reinsurance fund to the floor, a bill she introduced with Sen. Bill Nelson, D-Fla.

“After securing significant changes, as well as commitments to pass legislation to help lower health insurance premiums, I will cast my vote in support of the Senate tax reform bill,” Collins tweeted Friday.

But Democrats have repeatedly said that passing both bills would not negate the side effects of repealing the mandate. Insurers have warned that without the individual mandate they are likely to raise premiums for the country’s estimated 16 million Obamacare customers, or withdraw from the exchanges where some of these customers can purchase plans that are subsidized by the federal government.

Sen. Patty Murray, D-Wash., who helped author the bill that carries her name and that of Sen. Lamar Alexander, R-Tenn., said that her deal with Alexander wasn’t designed to mitigate mandate repeal.

“Our bill was designed to shore up the existing healthcare system and deal with problems that President Trump and Republicans have already created — not solve the new problems in this awful Republican tax bill,” Murray said on the Senate floor Friday.

Alexander-Murray makes cost-sharing reduction payments to insurers for two years. In exchange, states get more latitude to waive Obamacare insurer regulations. The payments reimburse insurers for a requirement to lower copays and deductibles for low-income Obamacare customers.

Trump ended the payments starting on Oct. 18. In response, many Obamacare insurers raised premiums to offset the costly requirement for lowering out-of-pocket costs for low-income customers.

Murray said the reinsurance bill sponsored by Collins and Nelson won’t offer relief.

“This bill is good policy on its own — but won’t stop the premium increases, coverage losses, and chaos that the Republican tax bill will cause,” she said Friday.

The nonpartisan Congressional Budget Office released an estimate Thursday that buttressed Murray’s claims. The agency found that Alexander-Murray doesn’t mitigate the impact on premiums or insurance coverage from repealing the mandate.

An earlier CBO estimate found repealing the mandate would lead to 13 million people not getting insurance over a decade and premium spikes of 10 percent each year.

However, a major point of contention between the two parties is the exact impact of the mandate.

Collins, who helped defeat a “skinny” repeal bill that included mandate repeal, told the Washington Examiner Thursday that the mandate inversely impacts middle-to-low-income Americans. She noted that 80 percent of people who paid the mandate last year were Americans earning $50,000 or less.

Another estimate from financial research firm Standard & Poor’s found that 3 million to 5 million people would go without insurance if the mandate goes away. After receiving backlash from Republicans about its findings, CBO has said it is re-evaluating the way it measures its impact of the mandate.

Health insurers have continued to press for the mandate, saying that some impetus is needed to bring healthier people into Obamacare plans to guarantee that they will be able to follow the law by covering people with pre-existing illnesses and not charge them more. They have stated that they oppose the repeal of the mandate, but have been open to a replacement, like the implementation of a waiting period.

States have the option of implementing such a mechanism, and the District of Columbia is taking steps to implement such a mechanism. The Massachusetts healthcare plan, after which much of Obamacare was modeled, already has an individual mandate. In Hawaii, the state has had an employer mandate for years that applies to employees who work at least 30 hours a week.

Obamacare repeal bills in the House and Senate often featured a mechanism to work similar to the mandate. For instance, the House passed a repeal bill back in May that tacked on a surcharge of 30 percent of a person’s premium if coverage lapses for more than 63 days.

Republicans have begun to stress that they see the different actions in Congress on healthcare as a lead-up to a replacement plan, though it’s not clear that Alexander-Murray and Collins-Nelson will gain the 60 votes in the Senate needed for passage. Both bills are expected to be brought up in a must-pass vehicle such as an end-of-year spending bill.

"Most of us look at this as a sort of runway to get to what we hope will eventually be something along the lines of Graham-Cassidy where we would do a more systemic fix for Obamacare that shifts money back to the states,” said Sen. John Thune of South Dakota, the Republican Conference chairman, referring to the name of a plan that would shift Obamacare’s revenues to states.

He continued, “In the meantime you've got to deal with the issues that are cropping up in the foreseeable future.”