The stinking derelict you have to step over on the subway platform may actually be a well-trained commando on a secret mission to keep you safe.

A team of elite Delta Force, Navy SEALS and Air Force operatives has been prowling the city streets in elaborate disguise to tighten up the city’s security before the Republican convention, The Post has learned.

The team, composed of special ops veterans, is concocting worst-case terror scenarios – including attacks by plane, truck, boat, missile and rooftop snipers, according to former special-operations commanders and an FBI terrorism joint task force agent.

The elite 12-strong unit – part of the larger so-called “Task Force of 400” – is trained to think “like the bad guys,” dreaming up new ways terrorists can wreak havoc on the city, then figuring out how to counter their plans.

For months the group has constructed scenarios for would-be assaults on Madison Square Garden, assassination attempts on President Bush and other party leaders, and possible suicide missions aimed at creating maximum chaos.

The group has donned disguises, posing as homeless people or dressing up as Wall Street financiers, to search for anyone who might be planning attacks.

Mike Kucharek, a spokesman for the U.S. Northern Command, confirmed the special-ops team is already in place. The Northern Command is part of the Defense Department and coordinates military assistance to civil authorities, as well as overseeing anti-terror task forces such as the one in New York.

“We are fully engaged for the Democrats and Republicans,” Kucharek said.

Sources said the counter-terror operation had been in place since early this year and could be passing along tips to the FBI and Secret Service, the federal agency in charge of security for the convention, which runs Aug. 30 to Sept. 2.

A similar team is also helping out in Boston, where the Democratic National Convention kicks off tomorrow, sources said.

“The big difference between law enforcement and military is that a guy in special ops is trained to look at things from the bad guy’s point of view,” said Terry Griswold, an anti-terror consultant and ex-chief of U.S. Army Special Forces in Europe.

” ‘How do I take out something?’ – that’s where they come from,” Griswold said.

A former Delta Force commander said the group was poring through convention schedules and routes to determine which might prove problematic.

Senior NYPD officials told The Post they knew nothing about the commandos, and a spokesman for Commissioner Ray Kelly denied there was any military involvement in the convention’s security plans.

But The Post’s sources confirmed the counter-terror team was indeed a presence. The sources said some of them had experience in Iraq and Afghanistan, and that most spoke several languages and were expert at spotting suspicious behavior. It’s not the first time such units have been used, the sources said.

On other occasions, similar teams have been sent to safeguard major events, including Super Bowls and World Series Games and last month’s G-8 summit.

By law, military personnel are forbidden from doing routine law-enforcement work in the United States, but gatherings dubbed National Special Security Events allows for the teams. The Republican convention was declared such last July.

CONVENTION-AL TACTICS

Is that really a tourist from the midwest wandering aimlessly outside MSG? Or is it a commando, here to identify security issues in anticipation of the Republican National Convention.

Who’s guarding the Garden?

NYPD

Secret Service

FBI

Delta Force, Seals, Air Force

NY State troopers

FEMA

MTA police

NJ Transit

Amtrak

How much does it cost?

Total: $76 million

Police overtime: $40-$50 million

Mobil bioterrorism lab: $16 million

Mobile command centers: $3.2 million

Other security gear: $10-$20 million

Who pays?

Congress: $50 million

New York City: $26 million