WASHINGTON -- The Senate's days-long debate on health care features a dynamic that's relatively rare on Capitol Hill.

Genuine suspense.

Debate kicked off Tuesday without an obvious endgame. Several Republicans voted to start debate but said the bill will have to be changed for them to vote to actually pass the legislation later this week. The amendment process promises to be extensive and freewheeling. And victory for Republicans and President Donald Trump is not guaranteed.

The Senate has started off by taking up the House-passed bill -- which doesn't have enough support to pass the Senate -- and it'll take near-unanimity among Republicans for them to alter the measure. Right now, they're deeply divided.

"We obviously don't have consensus on where we ought to go," said Sen. Ron Johnson, R-Wis. "No matter what we pass it's not going to fix the whole problem."

Here's a primer on how to watch this week's Senate debate on repealing and replacing the Obama health law.

FAST-TRACK PROCESS

First, the legislation is being debated under fast-track budget rules that allow it to pass on a simple majority instead of having to clear the 60-vote filibuster threshold required of other legislation. Debate is limited to 20 hours. Amendments, generally speaking, are unlimited -- and can be offered after debate time has expired in a Washington ritual known as "vote-a-rama." That's when amendment after amendment is voted on in what could be an all-night session on Thursday.

The first amendments get up to two hours of debate. During the voting marathon, debate is typically just two minutes.

AMENDMENTS GALORE

Unlike other bills, which typically are debated in ways that limit senators' rights to offer changes known as amendments, the current bill is wide open.

"I suspect there will be literally hundreds of amendments," said Sen. John Cornyn of Texas, the No. 2 Republican in the Senate.

The first amendment was offered by GOP leader Mitch McConnell of Kentucky. It is virtually identical to the version that passed the Senate in late 2015 that would repeal much of Obamacare and leave replacing it for later. It's sure to lose, even though it passed less than two years ago -- when skeptics of repealing the law without a clear plan for replacing it were assured of former President Barack Obama's veto.

Another McConnell amendment, likely to be swatted down by a parliamentary challenge by Democrats, includes the Senate's most recent "repeal and replace" bill -- scuttled last week for lack of support -- along with separate provisions from Sens. Ted Cruz, R-Texas, and Rob Portman, R-Ohio.

Democrats are poised to offer dozens of amendments of their own. For instance, they could try to eliminate tax cuts rewarding investors and upper bracket earners, just for starters.

One problem: Senators don't necessarily know how to draft amendments because they're unsure which bill they'll ultimately be amending.

PARLIAMENTARY PUZZLE

The special fast-track process, called reconciliation in Washington-speak, comes with tricky rules. Amendments that are carefully crafted and fit within the rules can pass on a simple majority vote. But many amendments run afoul of the Senate's byzantine rules, which mean they can require 60 votes and effectively be blocked by Democrats.

Among them is the so-called Byrd rule, named after former Sen. Robert Byrd, D-W.Va. It's complicated, but the Byrd rule disqualifies some of the GOP's ideas, such as a provision in the pending bill aimed at lowering premiums paid by younger, healthier consumers by allowing insurance companies to increase premiums paid by seniors.

The Byrd rule generally blocks provisions that don't affect the federal budget -- and blocks provisions whose changes to spending or taxes are "merely incidental" to a larger policy purpose. If such provisions are inserted despite the Byrd rule, any individual senators can knock them out with a point of order.

MCCONNELL'S LAST OPTION

At the very end of the debate, after dozens of votes on amendments and parliamentary challenges, majority Republicans can offer one, final substitute amendment. McConnell would probably be the author and it could represent one final grasp at consensus among fractured Republicans.

McConnell's last gambit could offer Republican senators a difficult choice since rejecting it would probably doom the whole effort. But consensus among Republicans has eluded McConnell for weeks, so there are plenty of reasons to be skeptical he can succeed now.

BACK TO THE HOUSE -- OR INTO CONFERENCE?

If the Senate should manage to manoeuvr its way through this week's legislative labyrinth, the resulting bill could go back to the House for a vote that would send it directly to Trump for his signature.

The other alternative would be to send the measure into official House-Senate negotiations known as a conference committee. Conference talks, insiders fear, could be a nightmare and invite balkanized Republicans to feud even more. In particular, tea party House Republicans and the Senate's more pragmatic GOP wing could be in for a fight.