Donald Trump’s Saturday speaking slot at the Conservative Political Action Conference is prompting an acrimonious backlash from the conservative critics desperately trying to mount a last-ditch campaign to block the GOP presidential front-runner from winning the party’s nomination.

A top aide to Trump rival Marco Rubio has accused CPAC organizers of being in the tank for Trump and clearing the way for his acceptance into mainstream conservatism, while an anti-Trump super PAC is pressuring organizers to rescind their invitation to the surging GOP frontrunner.

Potentially complicating matters further, sources tell POLITICO that Trump has made multiple donations ― including a $50,000 check last year ― to the American Conservative Union, the group that organizes CPAC. That dwarfs the amounts donated in recent years by allies of Trump’s rivals, all of whom are also scheduled to speak at the annual gathering, and seems likely to fuel already percolating suspicions among his opponents that the ACU has its thumb on the scale for Trump.

Even by the standards of CPAC, which over the decades has been in the middle of more than its share of contentious fights about the future of the conservative movement, the one brewing around this year’s gathering is shaping up as historic ― and historically nasty.