The mayors of several big cities, including Chicago, New York, Boston and Los Angeles, are joining together to support same-sex marriage rights, a move that could put additional pressure on President Barack Obama to do the same.

The effort is being organized by Freedom to Marry, an advocacy group that plans a news conference Friday in Washington, D.C.,, featuring mostly Democratic mayors and one Republican, Jerry Sanders of San Diego. Like others, Mr. Sanders has stated his views before, but organizers hope that their effort to join some 70 mayors together will give the issue a political boost.

“By joining the group, mayors hope to expand public and political support for ending the exclusion of same-sex couples from marriage,” the mayors’ group said in a statement.

The mayors planning to attend Friday’s news conference include Annise Parker of Houston, Thomas Menino of Boston, Antonio R. Villaraigosa of Los Angeles and Marilyn Strickland of Tacoma, Wash. Michael Bloomberg of New York and Rahm Emanuel of Chicago, Mr. Obama’s first White House chief of staff, have also joined the effort…