"We made a decision over the weekend, that while this presidential race for us is over, for me, and we will suspend our campaign today, we are not done fighting," he said.

Santorum, appearing with his family, told supporters that the battle to defeat President Barack Obama would go on but he pointedly made no mention or endorsement of Romney, whom he had derided as an unworthy standard-bearer for the GOP.

The former Pennsylvania senator stressed that he'd taken his presidential bid farther than anyone expected, calling his campaign "as improbable as any race that you will ever see for president."

"Against all odds," he said, "we won 11 states, millions of voters, millions of votes."

Santorum signaled his intention of maintaining a voice in the campaign to come, saying: "This game is a long, long, long way from over. We will continue to go out and fight and defeat President Barack Obama."