A Public Policy Institute of California survey on Tuesday showed Mr. Sanders with a lead of 32 percent to 14 percent against Mr. Biden, putting him in position to claim a clear majority of delegates from the nation’s largest delegate prize.

Mr. Bloomberg has shown more strength in recent state polls in the South, where he has been tied or ahead in recent polls of North Carolina, Virginia, Oklahoma and Arkansas. But Mr. Sanders has been highly competitive in these states and would still amass a meaningful number of delegates there.

The recent polls results also undermine the argument that Mr. Sanders has a hard ceiling on his support. He claimed a lead over Mr. Bloomberg, 57 percent to 37 percent, in a hypothetical one-on-one race in the NBC/WSJ poll, suggesting that his opposition might not inevitably coalesce around a single candidate, if one ever emerged.

Instead, the growth in Mr. Sanders’s support makes it easy to imagine how he could run away with the nomination over a divided field. Many of his opponents have an incentive to attack one another, rather than Mr. Sanders. And Mr. Bloomberg has been a focal point of attack this week, leaving Mr. Sanders relatively unscathed. Mr. Bloomberg might also be the focal point of the debate tonight.

The primary calendar offers Mr. Sanders’s rivals few natural opportunities to consolidate their support. Instead, he is heavily favored in Nevada, a state full of working-class, urban and Hispanic voters — the type who have generally preferred Mr. Sanders in the early states and national polls. And while South Carolina, with a large proportion of African-American voters, is less favorable to Mr. Sanders, the state could wind up further fracturing the field by propping up a faltering Mr. Biden or elevating an additional candidate, Tom Steyer, who held a mere 2 percent in the recent national polls.

For many candidates, the debates represent their only realistic opportunity to fundamentally improve their position. If anything, the effect of Mr. Bloomberg’s colossal ad spending — including the possibility that it could be turned to attack Mr. Sanders — looms as the single likeliest factor to reshape the trajectory of the race in the final two weeks ahead of Super Tuesday.

Yet at the same time, the polls suggest that the Democratic path to the presidency may be tougher than ever against President Trump, whose approval ratings have improved to well within striking distance of re-election. They have reached 45.8 percent among registered voters in the FiveThirtyEight tracker, while his disapproval has fallen to 50.1 percent.