For those who like statistics - see Government defeats in Lord since 1975.
From the outset, Brexit has been controversial (51.9% leave / 48.1% remain) and divisive (Scotland 62% remain, Northern Ireland 55.8% remain). The House of Lords has a key role in the constitution - explained here (pdf) - but its very existence is likely to be called into question in some quarters - see discussion at The Week. This is because the Lords have introduced what some politicians are portraying as anti-Brexit amendments. An alternative view is that the Lords has now called upon the elected House to properly address certain vital issues involved in achieving Brexit.
The Lords amendments address key issues such as whether the UK enters into some form of customs union with the EU, whether the UK seeks to retain membership of the European Economic Area (EEA) and whether sweeping powers are to be handed to Ministers to change the law by Henry VIII powers. These are key issues for the implementation of Brexit and the House of Commons will now have to face up to its role in addressing them. For example, if the Commons decides to support some form of Customs Union then that would bind the government to seek such an arrangement. On such vital questions, the House of Commons has so far been indecisive.
Reports forming a background to the Lords work on the Withdrawal Bill:
House of Lords Constitution Committee - The Great Repeal Bill and Delegated Powers - 9th report session 2016-17 (7th March 2017)
House of Lords Constitution Committee - Interim report on the EU (Withdrawal) Bill - 7th September 2017
House of Lords Constitution Committee - EU (Withdrawal) Bill Report - 29th January 2018 - This follows the preliminary (7th March 2017) and interim (7th September 2017) reports on the Bill.
Consideration of Lords amendments (the "ping pong" process):
Third reading of the Withdrawal Bill is scheduled for 16th May after which the Bill returns to the House of Commons where it is expected that the government will seek to reverse most, possibly all, of the Lords amendments. It will be for elected MPs to decide whether or not to bow to the government's will.
When a bill has passed through third reading in both Houses it is returned to the first House (where it started) for any amendments made by the second House to be considered.
If the Commons makes amendments to the bill, the Lords must consider them and either agree or disagree to the amendments or make alternative proposals.
If the Lords disagrees with any Commons amendments, or makes alternative proposals, then the bill is sent back to the Commons.
A bill may go back and forth between each House until both Houses reach agreement on the exact wording of the bill – this is known as ‘ping pong’
When the exact wording has been agreed by the Commons and the Lords, the bill is ready for royal assent. Once a bill receives royal assent it is made an Act of Parliament (the proposals in the bill become law).
Ping pong process
In exceptional cases, when the two Houses do not reach agreement, the bill falls. If certain conditions are met, the Commons can use the Parliament Acts to pass the bill, without the consent of the Lords, in the following session. A problem for the Withdrawal Bill is that, at the instigation of the government, the present Parliamentary session is two years - 2017-19. The need for the Withdrawal Bill to become law prior to Brexit (29th March 2019) means that the Parliament Acts 1911-49 cannot be used to override the Lords on this Bill.