One impact of Mr. Portman’s change of heart has been to fuel a movement seeking the repeal of a constitutional amendment in Ohio that bans same-sex marriage, according to the group leading the effort, FreedomOhio.

Requests to download petitions from the group’s Web site more than doubled overnight, to 50 a day, after Mr. Portman’s announcement, said Ian James, a co-founder of FreedomOhio. “His endorsement of freedom to marry, I would submit, has done more than any politician other than Barack Obama,” Mr. James said. …

On the other side, Phil Burress, president of Citizens for Community Values, said that he would now oppose Mr. Portman’s re-election “tooth and nail,” and that without the support of the 40 percent of evangelical voters in the Republican base, Mr. Portman, who will be a candidate again in 2016, “cannot win, he will not win.” …

The Republican establishment in the state stands behind Mr. Portman, and its leaders said that if he runs for a second term in three years, most voters will judge him on his support for lower taxes and spending, his longtime issues.