Factbox: Congress debates Obama's plan for Syria strikes

WASHINGTON (Reuters) - President Barack Obama’s request for congressional authorization for military strikes against Syria is set to dominate the Congress this week and next, with the outcome in doubt.

Obama is seeking backing for the strikes in response to an alleged chemical weapons attack near Damascus on August 21. Washington blames the Syrian government for the attack, which it says kills more than 1,400 people.

Here are the important steps in Congress:

- On Monday, Senate Majority Leader Harry Reid began moving to limit debate on the “motion to proceed” to a resolution authorizing the use of military force. The motion to proceed is the mechanism for bringing legislation before the full Senate.

It is not yet clear whether opponents will allow a simple majority vote on this procedural step or insist on a more difficult 60-vote majority in the 100-member chamber, in which Obama’s Democrats control 54 seats.

This first test vote is now set for Wednesday.

- On Monday evening, many members of Congress will receive a classified briefing from administration and military officials. For scores of lawmakers, this will be their first inside look at the Syria situation.

- Reid, in consultation with Senate Republican leader Mitch McConnell, will likely decide how many and which amendments to the bill will be debated on the Senate floor.

- Obama is due to lunch on Tuesday with Senate Democrats in an attempt to sway enough of them to support a military strike.

- He then plans a speech to the nation on Tuesday night, which could be a defining moment. Many undecided members of the Senate and House are likely to make up their minds on how they will vote after that speech and into Wednesday.

- If the Senate votes to debate the resolution itself, opponents could erect another procedural hurdle that would require 60 votes to limit this debate.

Should those votes prove necessary and if Reid succeeds in rounding them up, a vote on passing the Syria resolution could come on Thursday or Friday. A simple majority vote in favor of the bill would send it to the House for consideration.

- The House could simply vote for or against the Senate-passed bill. Or, it could amend the bill, pass it and send it back to the Senate for approval. Another option, though one that seems less likely now, is for a House committee to write its own bill for debate in the 435-member House. In any event, it could be next week before the full House takes action.

Reporting by Richard Cowan; Editing by David Brunnstrom