The new Congress was sworn in on Tuesday, and the first thing it did was prepare to repeal Obamacare.

Senate Budget Committee Chair Michael Enzi (R-WY) introduced a budget resolution Tuesday that includes "reconciliation instructions" that enable Congress to repeal Obamacare with a simple Senate majority. Passing a budget resolution that includes those instructions will mean that the legislation can pass through the budget reconciliation process, in which bills cannot be filibustered.

That means Republicans will only need 50 of their 52 members in the Senate, and a bare majority in the House, to pass legislation repealing the Affordable Care Act. According to the Wall Street Journal, the budget resolution could be passed by both houses as early as next week.

To be clear, passing the budget resolutions does not itself repeal Obamacare. But it’s the necessary first step if Republicans are to do that this year, and unless three or more Republican senators defect (or 24 House members do), it’ll be smooth sailing for the repeal effort from there on out.

Donald Trump has not yet taken office. But make no mistake: Congress is already starting to enact his, and the Republican Party’s, agenda, and Democrats don’t have the votes to stop it.

How the budget resolution enables Obamacare repeal

The idea that Republicans could junk Obamacare with a simple majority vote may sound baffling, given that Barack Obama famously had to wrangle together all 60 Senate Democrats in late 2009 to push the law through in the first place.

What makes this possible is that Republicans aren’t actually going to repeal all of Obamacare, as my colleague Sarah Kliff has explained. But they’re going to repeal enough of it to reverse almost all of the coverage gains made under it.

The reconciliation process, explained in detail here, can only be used to pass bills that affect spending and revenue — budgetary matters, in other words. It was created in the 1970s to make it easier for Congress to keep a budget, by giving the Senate tools to more easily change laws regulating big mandatory spending programs like Medicare, Medicaid, and the like.

Last year, Republicans passed the Restoring Americans’ Healthcare Freedom Reconciliation Act, a repeal bill that uses the reconciliation process. The Senate parliamentarian ruled that all the parts of Obamacare that it repealed — Obamacare's insurance subsidies, Medicaid expansion, the law’s tax increases, and its mandate to purchase coverage — could be dismantled through reconciliation.

The bill, introduced by Georgia Rep. and Health and Human Services Secretary-designate Tom Price, left some parts of Obamacare standing, like the requirement that insurers cover young adults through age 26. (That provision is very popular, and possibly harder to tackle through the reconciliation process.) It also left the requirement to cover Americans with preexisting conditions partially intact (you can read more about that from Sarah Kliff).

The reconciliation process normally can’t be used to pass legislation that increases the deficit 10 or more years into the future; that’s why the Bush tax cuts in 2001 expired after 10 years. Since Obamacare reduced the deficit, it would stand to reason that repealing it increases the deficit in the long run, and runs afoul of this rule. To get around this, the reconciliation bill preserves Obamacare’s cuts to Medicare doctor payments, and so is scored as reducing the deficit, because those cuts plus the cost of the insurance subsidies and Medicaid expansion swamp the revenue loss from repealing all of Obamacare’s taxes.

The human consequences of this legislation are immense. The Congressional Budget Office estimates that it would rip insurance away from 22 million people, mostly people getting coverage from the Medicaid expansion and who rely on subsidies in the insurance marketplaces.

The only thing that could stop Obamacare repeal is Republican opposition and/or disunity

There’s basically nothing that Democrats alone can do to stop this. Budget resolutions can’t be filibustered, so if Republicans vote to include reconciliation instructions for Obamacare repeal, there’s nothing the opposition can do about it. And, of course, they can’t filibuster the actual repeal bill; that’s the whole point of using reconciliation.

So if the plan doesn’t work, it’ll have to be because there are Republicans defections. While Republicans’ prospects for some kind of repeal are very good, a failure to reach total agreement within their own ranks isn’t impossible.

For one thing, before passing a repeal bill they have to decide when it’ll take effect. Price’s bill passed last Congress delayed repeal for two years, to give Congress time to work up a replacement before 22 million people lose insurance. But this time, they may drag out the process even longer. In early December, Senate Majority Whip John Cornyn (R-TX) floated the idea of delaying three years, until 2020. According to Bloomberg's Sahil Kapur, some Republican leaders are considering delaying until after the 2020 presidential election, while House conservatives are insisting on only two years’ delay.

If Republicans can’t agree on a timetable, and, say, a large bloc of conservative in the House or Senate refuse to vote for a bill with a delay longer than two years, that puts repeal’s near-term prospects in jeopardy.

The other question is whether Republicans have the votes for their plan to repeal now without first coming up with a replacement to cover the people currently covered under Obamacare. A number of Republican senators — including Lamar Alexander and Bob Corker of Tennessee, and Susan Collins of Maine — have voiced concern about repealing without simultaneously putting forward a replacement.

Alexander is the chair of the Senate Health, Education, Labor, and Pensions Committee, which will have to pass any repeal bill before it reaches the Senate floor; the “reconciliation instructions” in Enzi’s budget resolution refer specifically to his committee and the Senate Finance Committee.

Alexander’s opposition to this strategy would be a huge deal, and would pose a major obstacle for the repeal-and-delay plan. That being said, he’s sent mixed messages about this, saying early in December, “We’re going to begin immediately to repeal Obamacare and reconciliation is the only way to do it,” but then saying in mid-December, “Congress should replace and repeal at the same time, which requires figuring out how to replace it before fully repealing it.” If the latter sentiment wins out, then Republicans will need to figure out a way to get Alexander back on board if any repeal legislation is to pass.

Insisting on a replacement would seriously delay any effort to repeal Obamacare, as there’s nothing close to a consensus within the congressional GOP as to how to go about replacing the law. The Congressional Budget Office has declared that it will score legislation that replaces people’s comprehensive health insurance with much weaker catastrophic plans as equivalent to taking away their health insurance altogether.

That makes it likely that most Republican replacement plans, which rely heavily on making insurance skimpier and therefore cheaper, will be scored as taking insurance away from millions of people, spurring public opposition before they come up for a vote and risking still greater public outrage when they actually come into effect.

So repeal is not 100 percent guaranteed. But the only thing standing in its way is opposition or hesitance from within the Republican Party. For fans of Obamacare, that’s a very disturbing thought indeed.

Watch: Repealing Obamacare could change millions of lives