Like a blanket! Like a fog! That's how Niles Lathem covers this town, getting the big stories for the last of the old-time salty tabloids, which is to say Rupert Murdoch's New York Post.

He covers the White House, the State Department, the Pentagon, Congress and the CIA. All at once. Most major dailies have one person assigned to each of those beats, but if there are 8 million stories in this naked city, Niles Lathem wants to write all of them. In fact, if the Post wants to publish them, he has to write them all--he's the paper's only correspondent here.

On a given day, Lathem can have as many as seven bylines throughout the paper, according to amazed New York Daily News staffers, who one day pinned them all to a bureau bulletin board. Furthermore, he seems to be talking to everyone in Washington, quoting "top White House" officials, State Department aides and congressional sources, sometimes by name. He has put more than one Washington correspondent in the position of having to convince a worried editor that such tidbits as Nancy Reagan diseases or sultry spies or movie stars at media galas are, in fact, nonexistent.

One reporter claims Lathem told him that the key to covering Washington is "never stay at a briefing until the end."

"If he writes all the stories that have his name on them, he is the most amazing reporter since Clark Kent," says Lars-Erik Nelson, Washington bureau chief of the New York Daily News, Lathem's main competition here. The News has eight reporters in its Washington bureau.

"We feel that Niles is such a powerhouse in his performance of out-reporting the opposition--namely The New York Times and the New York Daily News--that we don't need any extra staffing," says Steve Dunleavy, the New York Post's metropolitan editor.

And it isn't just his omnivorous and ubiquitous reporting that makes him a "powerhouse"--it's his writing. Lathem is a living museum of a zippy style that illuminated the American press a half-century ago and is still popular in England. ("Are you going to mention Fleet Street?" Lathem asked when first approached about this story. "I did live in England--but I've never been to Fleet Street.") When President Reagan isn't "furious," he's delivering "thundering" speeches, all part of holding "the most powerful job on earth." "War clouds" descended on Lebanon twice last month, according to Lathem's stories.

Lathem has incensed everyone from the White House to the Nicaraguan Embassy to fellow journalists. Ordinarily this might be a sign of a media celebrity in the making, except for one thing: It seems that half the people who bluster about inaccuracies, sensationalism and Brit-shock reporting style have never laid eyes on him.

He's everywhere, he's nowhere.

This has caused problems. When Robert Redford was scheduled to attend a White House correspondents dinner, Lathem vividly described women at the dinner circling Redford's table--a good story, except that Redford had never shown up.

"I think if the New York Post was interested in covering the White House, they would send a reporter over here," White House deputy press secretary Larry Speakes fumed at a recent press briefing.

"Who is this guy Nils Lathem?" White House Deputy Chief of Staff Mike Deaver recently asked the first lady's press secretary, Sheila Tate.

"I never heard of him before and he never ever tried to call me for the story ," said Angela Saballos, a Nicaraguan Embassy press spokeswoman who, Lathem alleged in a news story, was not only a spy but a new "Mata Hari." (Lathem says he did call her.)

"After the story ran he called me," Saballos said. "But he didn't need me to do the story, so why did he need me after? It was a very annoying experience. Is this free press?"

Lathem, 27, affable and pudgy, doesn't understand the fuss. He was raised in Brooklyn and lived in England for a while as the son of a doctor employed at the U.S. Embassy. He went to Colorado College, then worked in New Jersey at the Paterson News and the Passaic Herald News. A former editor there, Miriam Taub (now with The Dallas Times Herald), recalled with affection, "He always wanted to do the big stuff without going through the basics."

He has been in Washington for two years, and for this article, he maintained his reputation as the invisible scoop artist by declining, after days of indecision, to be interviewed. Instead, he responded with a precise, neatly typed letter.

"I would like to say that I am here in Washington to do a job and not win a popularity contest with my so-called colleagues," he wrote. "I spend my days trying to get as many good stories as I can . . . I do not spend my time sitting around the White House press room waiting for handouts--or hanging around the Class Reunion a bar frequented by journalists moaning about pool assignments or other press office perks."

Invisible! He also refused to be photographed, instead furnishing a tiny snapshot about the size of a commemorative stamp.

Lathem has his defenders, some of whom attribute his sometimes abrasive and always aggressive style to his youth and his employer.

"He's young, fearless and doesn't know what he can't do," says Republican political consultant Roger Stone, who acknowledges he is one of Lathem's sources. "Also, don't lose sight of the fact that the kinds of stories he does are what his editors want and that's what sells the paper. He'd be fired if he turned in serious, boring stuff."

There are about as many Niles Lathem stories floating around Washington as there are news leaks.

* NEWS HOUND PANICS WALL STREET! The tale Niles watchers love to repeat is how he allegedly caused the stock market to drop 10 points one day after he called corporate chiefs arounds the country asking them about the story that Federal Reserve Chairman Paul Volcker would be ousted. (Lathem announced the ouster in April, two months before Volcker was reappointed.)

* MATA HARI WORKS WILES ON WASHINGTON! In his piece on Angela Saballos of the Nicaraguan Embassy, Lathem wrote that she had been trained by Cuban intelligence and suspected of being "at the center of an intricate Sandinista operation." This one got him the front page of the New York Post.

* MAD BOMBER HEADS FOR WHITE HOUSE! Last year, Lathem had Norman Mayer, the man who held the Washington Monument hostage for 12 hours, heading toward the White House when police gunned him down. Mayer's van had merely begun to roll down the slope on the Monument grounds when the police opened fire.

* NEW CANCER FEAR GRIPS NANCY! Lathem wrote that the first lady had had another cancer scare several months after it had been reported that she had a cancerous growth removed. In his daily press briefing, Larry Speakes not only denied it, but took the unusual step of attacking Lathem.

"I think it's a lousy reporting job on the part of the New York Post and their reporter Mr. Niles Lathem," Speakes told the briefing. "I think it's an outrage. I think it's an insult to decent journalists anywhere."

Asked if he was concerned that Speakes thought Lathem to be "unprofessional," Steve Dunleavy responded: "My dear, as far as I'm concerned, Niles will be in Washington long after Mr. Speakes remains an amusing memory."