As has been the case for some time now, Bernie Sanders is the front-runner in the Democratic primary, a position Tuesday night’s victory in New Hampshire only confirmed. While the race has just begun, the plausibility of his eventual nomination has already put a critical choice into view for his supporters. If victorious, Sanders will have to make the most difficult and potentially consequential vice-presidential pick of any major party candidate in recent memory. He will be 79 years old on Inauguration Day, which would make him the oldest president at the time of his swearing-in in American history—by nearly a decade. And although he’s made a lively recovery from the heart attack he suffered last year, it’s certain that his health and fitness will be not only major topics of the general election but also concerns at the top of Sanders’s own mind. He’ll want to choose a potential successor capable of ensuring the continuity of his agenda for the remainder of the administration and beyond.

Given this, it’s hard to imagine Sanders doing what many observers will insist he should: choose an obvious moderate who might reassure those voters who consider Sanders too left-wing to support. Yet it seems likely that Sanders will ultimately choose someone to his right; the pool of major and even secondary figures on the American political scene at his end of the ideological spectrum is incredibly small. Beyond ideology, Sanders would likely weigh a few other variables presidential candidates typically consider. It would be hard for him not to choose a running mate considerably younger than he is, even if he didn’t want to, but he could well decide to pick a youthful candidate, which might help him more with the kind of older voters backing Pete Buttigieg’s message of generational change than with young voters themselves. He could also choose a nominee who might boost his margins in a particularly important state or region—someone from the Rust Belt, white working-class heavy states of Hillary Clinton’s failed firewall in 2016; or from the diversifying South, where Democrats might still try to make gains if they’re feeling bold as the general election begins.

Representational politics would also weigh heavily on Sanders’s decision: Many progressives will insist, loudly and rightfully, that he ought to seriously consider picking a woman, a person of color, or both. Sanders has previously intimated that he’s leaning toward a female nominee, although he’s also declined to commit to any particular demographic profile. A diverse ticket could help ensure the breadth of the coalition that has emerged in the primaries is sustained through the general election, which will be critical if Sanders is to outperform Clinton in minority turnout within key swing states.

On top of it all, Sanders’s strategy for enacting his agenda has introduced a unique wrinkle into the equation. He’s been a critic of the push from progressives to fully eliminate the Senate filibuster, which he believes might be worth keeping in amended form to preserve some power for Senate minorities. To overcome the 60-vote threshold certain to hobble progressive policymaking even if Democrats win a Senate majority in November, Sanders has instead proposed taking full advantage of the vice president’s usually unutilized constitutional authority as president of the Senate, which would allow the White House to overrule the Senate parliamentarian’s judgments on the Senate’s rules for reconciliation bills, if not legislation in a broader sense. The last vice president to intervene in the Senate this way was Nelson Rockefeller, and the messiness of what ensued is captured in the Senate’s official biographyof him: