They've got 4 million followers.

He's got 16 billion simoleons.

This could get verrrrry interesting.

This being the stare-down between the National Rifle Association and Michael Bloomberg. Behemoth vs. behemoth.

Since the Reagan revolution, few lobbies have been mightier than the NRA. The Second Amendment defenders rarely lose on Capitol Hill, but lose they did just a few days ago, falling short in a sit-up-and-take-notice squeaker of a Senate vote on an amendment that would have let gun owners carry concealed weapons across most state lines. Score one for Bloomie and 450 of his closest mayor buddies (they call themselves Mayors Against Illegal Guns) . . . and their D.C. lobbyists and their consultants and their ad people. Score one against the NRA and the amendment's sponsor, that rising GOP star Sen. John Thune of South Dakota.

The Thune amendment inspired a gymnastic lobbying effort, juiced by Bloomberg's wealth -- he's injected $2.9 million of his own money into the mayors group since 2007, a spokesman confirms. Zooming in on how Bloomberg's group, far from an established Beltway force, out-lobbied the once-invincible NRA affords a glimpse of Capitol gamesmanship in the new Democratic-controlled Washington. There's a war room, of course, famous names dialing famous names and some in-your-face tactics, too.

And, naturally, there's enough bad blood left over to ensure many fights to come.

"The problem is that the NRA isn't willing to look at reasonable things," Bloomberg says in an interview. As for the Thune amendment that he just finished blocking, Bloomberg sums it up like this: "Another obstacle in the way of sanity."

In the opposite corner, the NRA's chief lobbyist, Chris W. Cox, calls Bloomberg an "elitist mayor with a big mouth . . . and a radical agenda . . . self-appointed mayor of America." And Thune's spokesman Kyle Downey says, "The fact that a guy who rides the New York subways with armed guards can tell the rest of America that they cannot defend themselves is absurd."

Things might never have gotten to this point if not for a seminal sit-down in April 2006 -- five months after Bloomberg was reelected -- at Gracie Mansion, New York's mayoral residence. Bloomberg and Boston Mayor Thomas Menino -- a natural ally as the New York mayor was born in Beantown -- invited about a dozen fellow mayors to talk about guns.

During his first term, Bloomberg had been introduced to the grimmest of mayoral rituals: late-night phone call, drive to the hospital, dead police officer, consoling the family.

"It brings it down to an emotional level when you actually look people in the eye, when you're in the room and see the body," Bloomberg says.