Labour, Brexit and the European elections
Leading figures in Labour like Jess Phillips were out on social media today, with a call to arms to fight the reactionary tendencies represented by The Brexit Party, promising to oppose with progressivism.

This is a difficult pitch for Labour. Labour is conceding the main declared objective of The Brexit Party - leaving the EU - the policy that defines the party now, and the achievement of which will give it impetus for its future goals.
In each scenario in which Labour gets to be the architect or co-architect of Brexit policy, Labour promises to deliver Brexit. If it wins a general election, it negotiates its own deal with the EU, and the UK leaves without a referendum. If it manages to negotiate a deal with the Tories, the deal comes back to Parliament, but not a referendum, and the UK leaves the EU.
If we look at what Labour want out of a potential exit deal, we see even more evidence that Labour concede ground to the reactionaries in the Brexit Party. They want to end Freedom of Movement! The fight they are putting up against the Tories, which will probably mean that no deal will be struck anyway, is over customs policy with 3rd party countries and allignment with regulations emanating from the single market. But the bit of Brexit that really agitates the reactionaries, the bit about not letting foreigners in, is singled out for Labour's approval.
Jess Phillips is really saying 'let's oppose with progressive policies, except for the main one which the racists want, which is to end freedom of movement' with her continued condoning of the Labour platform in the European elections.

Clive Lewis urged people not to vote tactically for explicitly pro Remain parties. Because doing so will not translate into seats, and may instead hand seats in the European Parliament to the Brexit Parties. But these are seats for MEPs that won't actually do anything. In fact, as explained above, in any scenario in which Labour get to be architects, the MEPs will be brought home again as we will leave the EU! 'Don't allow anyone to take Labour's MEP seats which our policy is to try and abolish as soon as possible a matter we are at this very moment trying to conclude!' Clive is urging.
At best, these elections are two things. First, a rubbish opinion poll on Brexit. Rubbish because of the historically low and politically unusual turnout; the special circumstances of this particular election; and Labour's attempt to be both Remain and Leave. Second, the elections serve as a campaigning rehearsal for a General Election, or a future referendum, honing infrastructure, support and branding.
The votes/seats mapping is not totally unimportant, because those who win seats can claim they won. But since those seats don't do anything else for the party, they are not decisive.
Philips and Lewis are intervening on the same page as Corbyn. The hope is to present Brexit - on which they want to equivocate to a certain degree - as a lower priority to a general left/right orientation. 'The real divide is not between Leave and Remain' he has said. On another occasion: “We could allow ourselves to be defined only as ‘remainers’ or ‘leavers’ labels that meant nothing to us only a few years ago. But where would that take us?" ... To which the obvious answer is: 'either out of the EU or not!'