Later in the town hall, Mr. Sanders was asked if he had any doubts about Mr. Biden’s “mental acuity.” Mr. Sanders did not address the question directly but questioned the length of Mr. Biden’s recent stump speeches.

“Joe was somewhere, where was he, Michigan or somewhere the other day, and he spoke for seven minutes,” said Mr. Sanders, whose events often run an hour or longer. “I don’t know how you say anything other than, you know, minimal discussion for seven minutes.”

Though six states vote on Tuesday, Michigan, where Mr. Sanders’s surprise victory in 2016 signaled the strength of his insurgent presidential campaign, has become a critical battleground for Mr. Sanders and Mr. Biden to show both their ability to turn out key blocs of support and to expand the electorate in a state that will undoubtedly be a major target for President Trump in the general election.

Mr. Sanders canceled events this weekend in Mississippi to focus his efforts on Michigan, effectively ceding another state in the South to Mr. Biden while redoubling his effort in Michigan.

Mr. Biden also picked up two major endorsements from national gun control advocacy groups, Everytown for Gun Safety and the Brady Campaign, evidence that Mr. Sanders’s past positions on gun control could undermine his support on an issue popular with many Democratic voters.

Mr. Booker, despite dropping out of the presidential race before the nation’s first primary contest, had campaigned in Detroit this cycle in an effort to pitch himself as the best candidate to re-energize the fractured Obama coalition and excite black voters in swing states that Hillary Clinton lost in 2016.

Though Mr. Biden has indicated that he would most likely select a woman as his running mate, Mr. Booker’s support for Mr. Biden will inevitably draw some speculation as to the New Jersey senator’s future.