Democratic delegates expressed alarm over president Joe Biden’s debate performance in private social media conversations that raised doubts about the viability of his candidacy. Politico obtained a cache of messages between California delegates from invite-only discussions held on Facebook and elsewhere showing heated exchanges about Biden’s appearance at last month’s first presidential debate, and DNC delegate Susan Bolle told participants that she’d heard from more than 150 local voters in the days afterward asking her to call for the president to step aside. “Obviously, the first step would…