The Obama administration's guarantee of a conviction in the trial of self-proclaimed architect of the Sept. 11 attacks Khalid Sheikh Mohammed could be used by the defense team to claim the jury pool has been tainted, says a former Defense Department official who calls the statements prejudicial.

Attorney General Eric Holder told a congressional committee on Wednesday that the case is going to federal court because he is certain a conviction will be reached.

"What I told the prosecutors and what I will tell you in what I spoke to them about is failure is not an option. These are cases that have to be won," Holder said.

But Charles "Cully" Stimson, a former deputy assistant defense secretary for detainees, told Fox News on Thursday that these statements play into the hands of the attorneys who will represent Mohammed and his four accomplices when they are returned to the scene of the crime for a federal trial.

"The defense will file a pre-trial motion before the judge and say, 'Your Honor, we respectively move to dismiss the charges," Stimson predicted. "The president himself and the attorney general, the top law enforcement official in the United States, have guaranteed conviction. What juror is not going to want not to at least fulfill the president's and the attorney general's desire?"

More On This...

The decision to prosecute the terror suspects on U.S. soil has sparked intense criticism from many who say the Sept. 11 attack was an act of war and a military tribunal is the appropriate venue for prosecution.

But President Obama and Holder have said bringing the suspects to New York offers the best chance of getting a conviction. 

"I have complete confidence in the American people and our legal traditions and...the tough prosecutors from New York who specialize in terrorism and have brought multiple convictions before are telling us that they will convict this person with the evidence they've got going through our system," Obama told NBC News this week in China.

While the guarantee may be an effort to reassure the public that the terror suspects will never be free, some worry it undermines the administration's stated goal of showcasing the federal justice system.

Defense attorneys for Mohammed can claim that the president and the attorney general have tainted the jury pool on a national basis and they could move to dismiss the charges, said Stimson, now a senior legal fellow at the Heritage Foundation.

It's unclear whether a federal judge would agree. However, Sen. Charles Grassley, R-Iowa, said Holder's remarks suggest a bit of bravado.

"I don't know how you can make a statement that failure to convict is not an option when you got juries in this country," he said. "I think a lot of Americans think OJ Simpson ought to be convicted of murder rather than being in jail for what he's in jail for now."

Sen. John Cornyn, R- Texas, who has sent a letter to the Justice Department and Homeland Security about the terror case, also challenged Holder about what to do if a jury acquits.

"Are you concerned that a judge may say you've made an election, an election to try these terrorist as criminal, and you're bound by that election and you cannot go back and revert to the laws of war in order to claim that you are indefinitley going to ... detain that individual? Are you worried about that?" he asked.

"No, I am not," Holder replied. "I think that under the congressional provisions that we have and the laws of the war, that we have an ability you cannot perhaps indefinitely detain somebody but you certainly can detain somebody for lawful reasons."

So confident are some in the federal justice system's ability to convict terrorists that Sen. Patrick Leahy, chairman of the Senate Judiciary Committee, said if the United States captures Usama bin Laden, there's no need to interrogate him.

During an appearance on "Washington Journal" on C-Span, Leahy said arguments raised by Republican senators about whether bin Laden would be afforded Miranda rights if he were captured amount to a "red herring."

"The red herring that my friend (South Carolina Sen.) Lindsey Graham was covering is not realistic," he said. "For one thing, capturing Usama bin Laden -- we've had enough on him, we don't need to interrogate him but the other point is why run scared from these murderers?"

Click here to see Leahy on "Washington Journal."

Fox News' Catherine Herridge contributed to this report.