Washington (CNN) Despite nearly a year's worth of heartache, Republicans can't quit trying to repeal Obamacare.

Senate Republicans plan to include a repeal of the individual mandate -- a key piece of the Affordable Care Act -- in their tax reform efforts. It's a move that dramatically raises the stakes for President Donald Trump and congressional Republicans, who are still looking for a single, substantial legislative accomplishment to deliver to the American people this year.

For Republicans, the benefits are tempting. If they can pull it off, it's a two-for-one victory. They will have overhauled the US tax code and taken a bite out of former President Barack Obama's signature legislative achievement, even if it falls short of the "root and branch" health care repeal many had vowed to see through.

But if they fail, Republicans will enter the midterm election year with their message muddled, without a victory on tax reform, and having spent a year's worth of time trying and repeatedly failing to dismantle Obamacare with nothing to show for it but bad headlines.

One Senate Republican, Susan Collins of Maine, warned her fellow party members to be wary of diving back into the health care fight and risk capsizing their tax reform efforts just as they were making progress.

"I personally think that it complicates tax reform," Collins told reporters as she left a GOP lunch Tuesday.

Repealing the individual mandate has been up for consideration for weeks, but the discussion had been happening mostly behind the scenes as Republicans debated using an estimated $338 billion in savings to cover the cost of and finance additional tax cuts over the next decade.

The mandate requires nearly all Americans to have health insurance or pay a penalty. But getting rid of it has a cost.

Roughly 4 million fewer people would be covered in the first year the repeal would take effect, the Congressional Budget Office said last week , rising to 13 million by 2027, as compared to current law.

Premiums would also rise by about 10% in most years of the decade, CBO said.

How it happened

Monday morning, President Donald Trump tweeted from a trip to Asia that he wanted to see the repeal of the individual mandate included.

I am proud of the Rep. House & Senate for working so hard on cutting taxes {& reform.} We're getting close! Now, how about ending the unfair & highly unpopular Indiv Mandate in OCare & reducing taxes even further? Cut top rate to 35% w/all of the rest going to middle income cuts? — Donald J. Trump (@realDonaldTrump) November 13, 2017

Senate leaders and many House rank-and-file members appear to have embraced the position.

"We had to get our members to where they were comfortable with it, and I think we are there," said Sen. John Thune, a Republican from South Dakota and member of GOP leadership.

"It just makes the overall bill a lot more possible," said Sen. Roger Wicker, R-Mississippi.

Introducing a repeal of the individual mandate into the tax debate, however, is a risky proposition for Senate Republicans who failed to repeal the mandate in July after three Republicans -- Collins, Sen. John McCain and Lisa Murkowski -- all voted against it

Even lawmakers who embraced repealing the individual mandate were keenly aware of the danger it could pose to their delicately crafted tax bill. After all, trying to re-write the health care law had proven to be a massive flop for Senate Republicans , a failure that has echoed across conservative talk radio and hurt the GOP with their donor class.

"I have some concern about mixing health care with taxes because the health care issues have been so difficult," said Sen. John Kennedy, a Republican from Louisiana who supports repealing the individual mandate as part of the tax bill. "But, it's really not health care that we are talking about. We are talking about tax."

A key driver of the Senate Republicans' decision Tuesday was simple arithmetic: They need the money. As it stands now, their tax plan increases the deficit outside the 10-year window and that is against Senate rules if lawmakers want to use a process that allows them to pass the tax bill with just a simple majority.

"Whatever people want to do is fine with me. My concern is that we not generate deficits, so if it is something we could use to buffer -- especially during the first 10 years -- deficits, then I'd be open to it," Tennessee Republican Sen. Bob Corker said. "As long as it's real. I know some people think it is a gimmick. I want to look into it more fully."

For now, Republican senators who voted against the repeal of the individual mandate in July, aren't coming out against the latest plan as of Tuesday evening. McCain and Murkowski have said they are both still examining the newest proposal, with McCain said he was leaning toward supporting it

JUST WATCHED Obamacare repeal: Watch McCain vote no Replay More Videos ... MUST WATCH Obamacare repeal: Watch McCain vote no 00:47

Democrats may bolt from tax talks

It's not just Republicans who will have to recalibrate their thoughts on the tax bill now that it's a hybrid tax-health care plan. The latest change could make it much harder to attract any Democratic support.

"This is turning a tax bill into a health care bill," Sen. Ron Wyden, D-Oregon, yelled during a markup in the Senate finance committee Tuesday.

"Republicans just can't help themselves," Senate Minority Leader Chuck Schumer said in a statement. "They're so determined to provide tax giveaways to the rich that they're willing to raise premiums on millions of middle-class Americans and kick 13 million people off their health care."

While getting Democrats to sign on was always a longshot, moderate Democrats met with White House officials last week at the Library of Congress.

The decision to include the repeal of the individual mandate will give Democrats a familiar arsenal of attack lines. In the days and weeks since Republicans first unveiled their tax bill, Democrats have tried to attack it as a tax cut for the wealthy and a major blow to the country's debt and deficit.

Those talking points have failed to create the kind of grassroots groundswell that was seen during the health care fight when Republicans endured hours-long town halls where individuals would come and recount stories about how repealing Obamacare would affect their daily lives. But that may change now.

Re-introducing the repeal of the individual mandate also brings groups like AARP, doctors and major insurance companies out of the wood work to oppose the Republicans' plan. On Tuesday, just after Republicans announced they would include a repeal of the individual mandate, a number of health groups stated their united opposition in a letter to Congress

JUST WATCHED Mnuchin pressed on Trump's false tax claim Replay More Videos ... MUST WATCH Mnuchin pressed on Trump's false tax claim 01:38

Will the House embrace it?

The news of the Senate change Tuesday reverberated across Capitol Hill where some House Republicans made a last-minute push to try to convince their leadership to include a repeal of the individual mandate in the House-version of the bill, something a few members had been pushing all along.

But, House Majority Leader Kevin McCarthy signaled Tuesday night that he didn't expect the House rules committee to make the change.

"If it's in the Senate, then there's the ability for conference," McCarthy said. "I don't think you'll see it in ours this week."

The view in the House is that the Senate must prove first it can actually pass a repeal of the individual mandate. The House already voted for it and passed it earlier this year.

"We didn't want to complicate tax reform and make it harder than it otherwise would be, and in fact, it was really, the Senate was the issue, so, we're now seeing if the Senate has the votes to actually repeal the individual mandates," House Speaker Paul Ryan said on a Fox News town hall Tuesday.

Kennedy said the Senate will get the job done.

"This is totally different than health care," Kennedy said. "Nobody is standing up and saying, 'If you do this, I'm not going to vote for the bill.' There has been none of that. Everyone wants to get to yes."