A late-arriving Rick Santorum fund-raising letter to Iowa voters is causing a stir for its harshly worded attack on presumptive Republican nominee Mitt Romney.

The letter, first reported on and reprinted in full by the Des Moines Register, arrived in Iowa mailboxes on Monday, nearly a week after the former Pennsylvania senator suspended his campaign.

"It truly frightens me to think what'll happen if Mitt Romney is the nominee," reads the undated letter, which is signed by Santorum and paid for by his campaign.

It goes on to repeat some of Santorum's most frequent attacks on Romney, calling him "a Massachusetts moderate," and suggesting that "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's "blunt message," said the Register, had Iowa Republicans speculating whether its unfortunate timing was something more than a simple mistake. "Or, did Santorum, who is still raising money and possibly not quite ready to come to grips with his loss, send this wording out deliberately?"

A Santorum spokeswoman said Tuesday night that the mailer was paid for and sent out before Santorum quit the race.

While he is widely expected to endorse Romney at some point, Santorum has so far declined to do so. In a conference call with reporters on Monday, Santorum held off on backing his former rival said he had not spoken with him since leaving the race.

"I haven't supported any candidate at this point," Santorum said.