The Democratic presidential candidates covered a lot of ground during their debate Tuesday, but one issue received little attention: their theory of political change. How exactly would Hillary Rodham Clinton or her rivals pass the programs and proposals they advocate?

The problem is simple. President Obama’s experience during his first term shows that efforts to find common ground with Republicans are unlikely to be successful. Since then, Mr. Obama has taken a more combative approach, but the results have been similar. Absent the big Democratic majorities of 2009-2010, the party’s legislative agenda has gone nowhere. Democrats can still block Republican proposals, but their own bills are largely dead on arrival in Congress (at least unless dissidents in the G.O.P. undermine the party’s control over that chamber).

Democrats’ options for changing the status quo are limited. The simplest way that a president can produce legislative change is by taking back the White House from the other party, which makes it possible to enact bills that would otherwise be vetoed. That option, of course, is off the table for Democrats in this election, just as it was for Mr. Obama in his 2012 campaign, when he made the now-disproved claim that his re-election would “break the fever” among Republicans.