Rick Santorum's 2012 bid for the Republican presidential nomination may have ended last week, but negative campaign mailers aimed at Mitt Romney are still arriving in supporters' mailboxes, according to the Des Moines Register.

“It truly frightens me to think what’ll happen if Mitt Romney is the nominee,” said a signed letter from Santorum's campaign that first appeared in Iowa mailboxes Monday. "My friend, Republicans and conservatives will be crippled by a nominee who presents zero contrast with Barack Obama on the major issues of this election."

The letter caused conservative radio talk show host Steve Deace to speculate on his show about whether it mailed knowing it would arrive after Santorum dropped out.

“One [explanation], this is simple campaign ineptitude. Two, there is conflict between Santorum’s personal convictions and those within his campaign who would like their careers extended by Romney come the general election,” Deace said. “Three, this is simply an attempt to leverage more out of Romney by reminding him his base likes him less than they liked McCain, and four, all of the above."

Deace's radio show airs in Iowa, North Carolina, Kentucky, Virginia and several other states.

Santorum has scrubbed his website clean of all anti-Romney content. Former presidential candidate Jon Huntsman did the same thing after dropping out in January, but he also immediately turned around and endorsed Romney.

During a fundraising conference call Monday night with supporters, Santorum declined to endorse the former Massachusetts governor.

“I haven't had a chance yet to talk to Governor Romney, but we'll be talking to both of them and we’re going to go out and do what we believe is in the best interest of our country,” Santorum told about 4,000 participants on the call, according to National Journal.

Related on HuffPost: