James A. Johnson, 46, has a reputation as an unflappable, buttoned-down manager, but when he called friends yesterday to tell them of his selection as next chairman of the Federal National Mortgage Association, he couldn't hide his excitement.

"He's very cool and unemotive," said former Vice President Walter F. Mondale, who gave Johnson his start in national politics. "But I felt some sweat coming through the phone today."

Johnson's elevation to chairman of Fannie Mae underscores the swift and successful transition he made from politics to finance. It is likely he will be called on to meld his political skills and connections with his newer talents as an investment banker as he enters what could be a difficult period for all financial institutions -- particularly government-chartered enterprises such as Fannie Mae, which the Bush administration has proposed come under stricter supervision.

"He understands the political process, which is something Fannie Mae will be into the next few years," said one person who has known him well for many years. "At a time when Fannie Mae has a lot going on Capitol Hill, his ability to understand political institutions is a very important piece of things."

Six years ago, he was the chairman of Mondale's presidential campaign, after serving earlier as executive assistant to Mondale in the vice president's office. But when Mondale lost the 1984 election in a landslide to Ronald Reagan, Johnson quickly shifted gears, joining Shearson Lehman Hutton (now Lehman Brothers Inc.) as a managing partner.

One of his principal clients while at Shearson Lehman was Fannie Mae, but it wasn't until last January that he joined the mortgage institution full time as vice chairman.

Friends describe him in superlatives. "He's a spectacular manager, an anticipatory manager, a guy who thinks four steps ahead of everybody," said John Reilly, a Washington attorney.

"He's a very calm, cool and methodical person, probably the best organized I've ever seen," said William Daley, president of Amalgamated Bank in Chicago.

"Jim is highly unusual in a number of ways, but particularly in that he made the transition from government life to investment banking very successfully," said Thomas Schick, an executive vice president at Lehman Brothers. "He gained the acceptance of his peers very quickly and became one of our more senior and respected bankers."

Johnson said he was attracted to Fannie Mae because of the "challenge of being in a very major business with the opportunity to make a real difference in the housing" of the country.

Johnson declined yesterday to talk about changes he might institute once he becomes chairman on Feb. 1.

But in a telephone interview he talked about his own management style. "I believe very strongly that management style flows from strategy," he said. "I believe the fundamental challenge of the chairman of any company is to set the strategy and see the challenges that face the company."

Johnson likes to immerse himself in the details of whatever he manages, whether that is knowing the voting patterns of all 99 counties in Iowa or the details of putting together a deal on Wall Street.

But his background as a political operative also taught him the importance of quick decision making. "A lot of people who come from government are process oriented," Schick said. "Jim was very much transaction oriented and 'get-it-done' oriented."

Johnson grew up in Democratic politics in Minnesota, where his father was speaker of the House in the 1950s. He worked for Mondale in the Senate and in the vice president's office.

In 1981 he founded a consulting firm with Carter administration official Richard Holbrooke. Called Public Strategies, the firm offered strategic advice to corporations and served as the backdrop for Johnson's eventual move to the Mondale campaign. After the 1984 election, Johnson joined Shearson Lehman Hutton.

Friends on Wall Street say Johnson prospered as a deal maker, particularly in his specialty of privatization, in which shares of a state-owned company or agency are sold to private investors. That gives him special expertise in the arena where Fannie Mae is a major player.

At Shearson, he also developed relationships that will be important in the months ahead, including one with then-Shearson colleague Richard A. Darman, now director of the Office of Management and Budget.