The two far-right eurosceptic groups among the eight in the current parliament would see their share rise to 14 percent from 10 percent, despite the loss of Brexit campaigners the UK Independence Party. That reflects gains for Italy's League, adding 21 seats, Germany's AfD, gaining 11, and Marine Le Pen's French National Rally, which would add six seats if polls hold.
However, realignments of existing groups are likely after voting ends on May 26 and before the new parliament sits on July 2 as national parties seek allies that fit their policies and can leverage their strength with funding and committee posts.
Italy's 5-Star movement, in government with the League, sits now with UKIP but has looked at joining groups further left in the chamber. The polls suggest it could gain eight seats to 22 in May, but those may not, in fact, bolster the far-right.
There are also question marks over the alignment of some 24 seats for Poland's ruling Law and Justice party, often hostile to Brussels, as its ECR allies the British Conservatives depart.
Also unclear are the 18 French seats which polls suggest President Emmanuel Macron's En Marche movement may win.
Adding them to the centrist ALDE, home to some Macron allies and which shares Macron's strongly pro-EU line, would give ALDE 93 seats, making it easily the third biggest bloc. But Macron has been wary of confirming which alliances he will make as he looks to use the May elections to resist eurosceptic forces.
One consequence of uncertainty over the make-up of the new parliament -- which might also be upset by a delay to Brexit -- could be delay in forming the new executive.