WASHINGTON (AP) - President Barack Obama's campaign manager says Republican Mitt Romney has been able to grind down his opponents for the GOP presidential nomination but won't be able to "buy the presidency."

Campaign leader Jim Messina issued a written statement reacting to Republican Rick Santorum's withdrawal from the race without ever mentioning Santorum or his decision. Messina instead referred to "developments in the campaign" and focused a sharp critique on Romney, who is now essentially cleared for the GOP nomination.

In doing so, Messina signaled the Obama campaign was immediately escalating its offensive on Romney.

Messina said it was not surprising that Romney outlasted his Republican competition with negative ads.

He said the more the American people see of Romney, "the less they like him and the less they trust him."