The "moderate" label was always a poor fit for Biden, used mostly as a facile shorthand to denote that his politics differed from those of Bernie Sanders and Elizabeth Warren. As I observed after his selection of Kamala Harris last August, Biden's actual career-long persona is "regular Democrat," with issue positions and priorities that have evolved over the decades in parallel with the mainstream of his party. The appointment of progressives to multiple administration positions—especially at the sub-cabinet and agency levels—and the ambition of Biden's major proposals in a range of policy areas can be viewed as a simple reflection of a recent leftward shift within the Democratic Party as a whole.
But there's another change that's important here. Biden and his team aren't just being pulled by internal partisan dynamics toward a strengthening left wing. They are also much less motivated than previous Democratic administrations to court the approval of the ideological center-right.
Both Bill Clinton and Barack Obama appointed Republicans to their administrations and pursued bipartisan policy solutions in part to build reputations among political elites as mature statesmen capable of rising above mere partisan loyalties. Their primary audience for these decisions was an "official Washington" composed of media thought leaders, think tank staffers, executives and lobbyists, and other political actors who had become accustomed to view Republican rule of the executive branch as the natural order of things (Republicans held the presidency for all but 4 years between 1968 and 1992, and all but 12 between 1968 and 2008) and to venerate right-of-center figures like Colin Powell and Alan Greenspan as the ideal personifications of public service. Much of this official Washington applauded Clinton's and Obama's "responsibility" and "toughness" when they departed from traditional liberal doctrine to pursue deficit reduction at home or military intervention abroad, and sympathetically portrayed non-Democrats within their administrations like David Gergen, Bill Cohen, and Robert Gates as the adults in the room keeping a necessary check on liberal naïveté.
Expectations that the Biden presidency would follow a similar path had caused a certain anticipatory disaffection on the left. But Democratic veterans of the Clinton and early Obama eras see a very different Republican Party when they look across the aisle today. The last decade of American politics, marked by the Tea Party movement and ascendancy of Donald Trump, has convinced even "establishment" Democrats that making concessions outside their party doesn't provide them much benefit—for either producing major policy achievements or realizing significant political advantages. And official Washington has discarded the once-prevalent assumption that the Republican Party is a valuable source of expertise and experience upon which Democratic presidents should productively draw. After the last four years, which party has a better claim to be the adults in the room?
If Biden had defeated Trump by a landslide margin in November, it might have paradoxically tempted him to govern in a more cautious style as a way to keep a wave of new defectors from the GOP inside his coalitional tent. But the last two elections have demonstrated that the anti-Trump Republicans who populate high-status editorial pages and Beltway professional circles are not representative of a numerically populous mass constituency; their approval might make a positive impression within a limited peer group, but it simply doesn't sway many voters. As long as the Republican Party remains devoted to Trump's style of politics if not Trump himself, Democrats may calculate that moderates and conservatives who find Trumpism intolerable will have little choice but to root for Biden's success even if his record is more liberal than they'd like.
The new opportunities for influence granted to the American left by the decaying position of the center-right may turn out to be one of the most unexpected political legacies of the Trump years. Center-right elites used to see themselves as the natural leaders of a "center-right nation." But today they are increasingly abandoned by both partisan sides, facing the realization that they speak mostly for themselves.