The Senate is expected to vote this week on an Obamacare repeal package designed to put them — and President Obama — on record rejecting the president’s signature domestic achievement.

It’s the first time Congress — with Republicans in the majority of both chambers — will have approved a rollback of the major portions of Obamacare.

If the repeal passes as part of budget reconciliation — it needs a simple majority of 51 senators for approval  — Obama is expected to veto it. But since Republicans recaptured the Senate majority in 2014, Senate Majority Leader Mitch McConnell (R-Ky.) promised he would use reconciliation to give Republicans their say on Obamacare and force a veto fight. But even if the bill lands on the president’s desk, it’s  unlikely the GOP-controlled Congress has the votes to override a veto.

The bill also contains language to defund Planned Parenthood, despite the killings at a Colorado clinic last week.

“By the time we are done, the legislation the Senate passes will eliminate more than $1 trillion in tax increases placed on the American people, while saving more than $500 billion in spending,” said Senate Budget Committee chairman Mike Enzi (R-Wyo.).

But first the Senate will embark on what’s known as a “vote-o-rama,” a marathon session that is expected to kick off on Tuesday night and will likely last much of the week. There will be an entire day of debate followed by an unlimited number of amendment votes. But there is a limit: lawmakers will have to move on to renewing the highway bill before it expires on Friday.

Also looming in the near future is the Dec. 11 deadline to keep open the government. Leaders hope that by including the more controversial language on Obamacare and Planned Parenthood in the reconciliation bill, they can stave off a conservative revolt to attach that language to the so-called omnibus. It’s unclear whether that tactic will work.

The Senate is scheduled to recess for the year on Dec. 18, leaving fewer than three weeks  to pass a spending bill, extend nearly 50 expired tax breaks, pay for highway construction and reauthorize benefits for 9/11 first responders.

Here are five things to know about how this Obamacare debate plays out:

1. Nobody expects this bill to become law. Senate Republicans have cheered the reconciliation bill as their best chance to pass Obamacare repeal language this Congress but even they don’t believe it will become law. But it’s important political symbolism with the 2016 elections looming and Republicans, now in control of both chambers of Congress, having repeatedly vowed to repeal Obamacare.

The White House warned Congress in October that Obama would veto the House-passed version of the bill and that’s exactly what Republicans want. It gives them a great talking point on the campaign trail.

“It’s important I think for the American people to understand that the public still stands strong and united against the Obamacare legislation,” McConnell said Tuesday. “Obviously we’re not anticipating a presidential signature but I think the president should have to take credit for the debacle that this legislation has created.”

The veto fight allows Republicans to say Obama ignored the will of Congress in order to protect a treasured domestic achievement. Republicans want to say they did everything in their power to make good on promises to repeal the law.

2. The bill can pass with just 51 votes Reconciliation bills need only a simple majority to be approved —  meaning Republicans could force Obama into a veto fight without the help of a single Democrat.

The House has voted dozens of times to repeal all or part of Obamacare but the Senate has never been able to surmount the typical 60-vote hurdle needed to pass most bills.

Senate leaders spent weeks tinkering with the bill to ensure that it would  not violate the “Byrd Rule,” which limits language in reconciliation bills to policies that have a direct budgetary impact. The parliamentarian decided last month that a full Obamacare repeal would not be possible so budget committee staffers scaled back some areas of the repeal language.

Presidential candidates Sens. Ted Cruz (R-Texas) and Marco Rubio (R-Fla.) joined with Sen. Mike Lee (R-Utah) in threatening to oppose the bill because it does not fully repeal Obamacare.

Leaders reworked the bill again in recent days and Lee said Monday that the new agreement on reconciliation looked good. Most Republicans say they are confident the bill can pass, putting them on record sufficiently dismantling Obamacare.

“The architecture of Obamacare will go away with this vote.” said Senate Republican Conference Chairman  John Thune (R-S.D.).

3. Senators will debate for 20 hours before any amendment votes begin. Senate rules require 20 hours of debate before the amendment process can begin. Aides said they hoped to start that process before the end of the day on Tuesday in order to start debate by early Thursday morning.

4. Senators can offer an unlimited number of amendments. Senate rules allow lawmakers to offer unlimited amendments on budget bills. Getting started early is critical because Congress still has to pass a government funding bill to prevent a government shutdown before Dec. 11. And the amendment process could take hours, if not days.

Budget amendment votes are stacked back-t0-back in a process known as a vote-a-rama. Senators have called the system everything from “dumb”  and “awful” to “grueling” but that doesn’t usually make things go any quicker. The Senate spent nearly 16 hours voting on amendments to the budget resolution in March.

Fortunately, reconciliation rules dictate that amendments are subject to the Byrd rule, and some amendments could be dismissed as non-germane.

It is also possible that members aren’t as excited about amendments as they were the last time.

“I think the appetite for a lot of extraneous amendments may not be the same as it has been in the past,” Thune said. “Its a vote-a-rama. It is one of the weirdest creatures I know around here.”

5. Congress probably doesn’t have enough votes to override Obama’s veto. Republicans would need the support of tw0-thirds of both the House and the Senate to override a presidential veto.

Leaders would have to rally support from 13 Democrats to reach that unlikely bar. Congress has been able to override just 4.3 percent of all presidential vetoes since 1789.