Socialists are proving to be a difficult voting bloc of presumptive 2020 Democratic presidential nominee Joe Biden to wrangle as he tries to knit together a coalition that will defeat President Trump in the fall.

Leaders of Democratic Socialists of America this weekend announced the organization wouldn’t be endorsing Biden as he embarks on his general election battle against the White House incumbent. The group had backed Bernie Sanders, Biden’s last remaining Democratic rival, but the Vermont senator last week dropped out of primary contention.