Senate Majority Leader Harry Reid joined in on a potentially bruising presidential battle Tuesday by opening up a second front of the Democratic assault on Sen. John McCain, R-Ariz., the front-runner for the Republican presidential nomination.

Reid's battle centers on the detainee torture debate, in which McCain — a former Vietnam POW — fought the Bush administration against curbs on the use of harsh interrogation methods. When the dust finally settled on the issue two years ago, the Bush administration won a major concession: The bans were limited only to the military, not other agencies such as the CIA.

Now, behind closed doors, Democrats have added in a measure popular among the Democratic caucus to a conference report on a classified intelligence bill. The addition would reopen the debate as well as fresh wounds among Republicans who opposed the measure and McCain, who's trying to solidify their support behind his presidential candidacy.

The measure — which must be passed by both the House and the Senate to take hold — would ban all forms of interrogation not approved by the Army Field Manual, the guide for all military servicemen and women, as well as those who work for agencies that fall under the Defense Department.

The provision eliminates harsh interrogation techniques like waterboarding, something Democrats have lately tried to get Attorney General Michael Mukasey to outlaw. The measure was added during House-Senate negotiations by a one-vote margin.

In crafting the Detainee Treatment Act of 2005, McCain, and a small handful of influential Republicans, sought to have this same ban included in their bill. Vice President Dick Cheney and other administration officials successfully defended a separate policy for the CIA, and the agency was excluded in the measure before it hit President Bush's desk.

The Senate scheduled a vote Wednesday to close off debate and avoid a bill-killing filibuster. Sixty votes were needed to prevail. It is unclear on which side McCain will fall — or even whether he will be present for the vote. He needs the support of conservatives in his presidential bid, but this is another cause on which they and McCain disagree.

McCain's spokeswoman had no comment. But Reid, D-Nev., appears to be laying down the gauntlet, while at the same time signaling acknowledgement of McCain's nomination despite the fact that he's still opposed by former Arkansas Gov. Mike Huckabee for the GOP bid.

"It's up to the Republicans (to pass this bill). We need, as you know, nine Republicans to support us on this. I hope that they will follow Senator McCain's example — at least, that's what he's indicated, that he's against torture," Reid said Tuesday.

He added: "Let's see if he can pick up — out of his 49, let's see if he can pick up nine. That would mean he'd have to be here too, you know," referring to the number of votes McCain would have to muster.

A Reid spokesman said this vote will be seen as "the substantive vote" on the measure. "The question is whether this country is going to condone torture or not," the Reid spokesman said.

The vote is very likely to fail, but by a narrow margin.

Republican sources told FOX News that Cheney, attending the weekly Tuesday party lunch for Republicans, talked to members about the bill and expressed concern about this provision.

One one senior Democratic leadership aide said, "We know some Republicans want to vote with us, but they're waiting to see what John McCain does."

And the aide said that if Democrats fail Wednesday, they'll be glad to take up the matter again later in the year — when it would be even more of a thorn in McCain's presidential aspirations.