The uncertainty in the political calendar has rippled around the country. Also on Wednesday, West Virginia became the 15th state to postpone its nominating contest, from May 12 to June 9, bringing the number of states that have done so to 15.
In Wisconsin, where the primary is scheduled for April 7, Republican lawmakers have blocked a proposal from Gov. Tony Evers to send out mailed ballots. Mr. Evers is planning to deploy the National Guard to work at polling places.
In a statement, Mr. Sanders said that the state should follow the advice of public health experts to promote social distancing.
“People should not be forced to put their lives on the line to vote,” Mr. Sanders said.
Before the coronavirus upended the campaign, Sanders aides were looking toward Wisconsin as a key state for the senator. But a poll published Wednesday showed Mr. Biden with a big lead there.
Reporting was contributed by Michael Cooper, Alan Blinder, Karen Zraick, Margot Sanger-Katz, Motoko Rich, Peter Baker, Sarah Mervosh, Tariro Mzezewa, Katie Rogers, Patricia Mazzei, Maggie Haberman, Stephanie Saul, Sydney Ember, Dave Montgomery, Manny Fernandez, Thomas Gibbons-Neff, Farnaz Fassihi, Marc Santora, Megan Specia, Iliana Magra, Zolan Kanno-Youngs, Somini Sengupta, Ernesto Londoño, Apoorva Mandavilli, Mike Baker, Sheri Fink, Joseph Goldstein, Michael Rothfeld, Benjamin Weiser, Alan Yuhas, Abby Goodnough, Knvul Sheikh, Manuela Andreoni, Letícia Casado, Laura Chang, Denise Grady, Austin Ramzy, Keith Bradsher, Andrew Das, Michael D. Shear, Elian Peltier, Raphael Minder, David D. Kirkpatrick, Kate Kelly, Peter Eavis, Mujib Mashal, Matt Apuzzo, Christopher Clarey, Mohammed Hadi, Erin Griffiths, Tara Parker-Pope, Andy Newman, and Chris Horton.