Rick Santorum said Thursday he dropped out of the presidential race because his fundraising tanked in April as his own supporters told him the race was over and the onetime Pennsylvania senator needed to back GOP rival Mitt Romney.

Speaking to the Family Research Council’s “Washington Watch Radio,” Mr. Santorum said his campaign had gone too deeply into debt for him to continue, and said once he lost the Wisconsin primary to Mr. Romney it became impossible to go on.

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“The last week after Wisconsin, we basically raised almost no money. We had solicitations going out and people emailing back and saying, ‘The race is over, you’ve got to join the crew,’” Mr. Santorum said.

He also said the delegate math made it clear Mr. Romney was going to win the nomination outright at the GOP’s convention in Tampa in August.

Mr. Santorum, a former two-term senator from Pennsylvania, said he was gratified to see a grassroots conservative movement propel him — though he said if conservatives had unified earlier in the race things might have turned out differently.

By the time conservatives began backing him en masse, he said the campaign narrative was already locked into horse-race questions about which candidate was ahead.

“I got the question ‘When are you getting out?’ more than I got ‘Hello,’” he said. “It was just this drumbeat that you were hurting that party.”