Vermont Sen. Bernie Sanders’ Presidential campaign is moving a scheduled speech in Phoenix to a bigger venue, to meet surging demand for tickets. The speech, scheduled for Saturday evening, will now be held in the city’s Convention Center, in a room that can accommodate more than 10,000 attendees.

Last week, Republican candidate Donald Trump also switched a planned speech to the Phoenix Convention Center, after strong demand. The Trump venue, however, was in a smaller part of the Convention Center, designed to hold 5,000 or so attendees.

The change of venue is the latest sign that Sanders, a self-avowed socialist, inexplicably, is generating enormous interest among the hardened progressive left of the Democrat party.

The speech coincides with a meeting of NetRoots Nation, a national conference of left-wing bloggers and activists. Sanders will participate in a town-hall style meeting at the conference, along with fellow candidate, former Maryland Gov. Martin O’Malley.

The apparent surge in support, or at least interest, for Sanders says more about dissatisfaction within the Democrat base with frontrunner Hillary Clinton than the iconoclast from Vermont. At his best moments, Sanders seems more like a popular college radical than elder statesman of the party.

His speech will cover the usual leftist territory of money in politics, climate change, income inequality and the rising cost of college. This agenda could have been lifted from any leftist meeting over the past two decades, so it is hard to argue that Sanders is tapping into some new or unconventional mix of issues.

Hillary Clinton herself usually covers the waterfront of these topics in her stump speeches. In a recent Fox News poll, however, almost one-third of Democrats said Clinton’s natural instinct was to “hide the truth.”

Sanders rise, the strong demand for him in Phoenix and recently added campaign events in Texas, one of the early primary states, is no doubt related to this finding.

Hillary will no doubt dispatch with Sanders’ primary challenge. Dispatching with the doubt that is fueling is rise is a much more difficult challenge.