He wears blue jeans and plaid shirts in the executive mansion. He preaches government reorganization and a balanced budget. He is informal and effective with the press. His aides talk about the new spirit of openness and honesty he has brought to government. His critics say he is long on style and short on obstance.

No, this is not a description of Jimmy Carter, President of the United States. It is a description of James R. (Big Jim) Thompson, governor of Illinois, who in appearance and personality resembles the President about as much as Billy Carter resembles Billy Graham.

Thompson, all 6 feet 6 inches of him, nevertheless, welcomes the Carter comparisons, because he is seriously interested in becoming a national political figure. While he draws a distinction between national policking and actually running for president, Thompson does not shrink from the thought that he might wind up as the Republican presidential nominee.

"I think it would be wrong for the governor of the largest state under Republican control to refrain from entering national Republican politcs, and I don't intend to refrain," Thompson said blandly in an interview.

"This is not the equivalent of running for the presidency, in my mind. And as far as I can tell, the people of Illinois are not put off by presidential talk, they promote it. I've got to dampen their ardor. When I walk through shopping centers people say I want you autograph, you're going to be President."

Unafflicted by either bashfulness or lack of candor, Thompson says openly that he will travel around the country after the legislature adjourns and make himself known to prospective voters and prospective Republican delegates. One gets the impression he can hardly wait.

The 41-year-old former U.S. attorney in Chicago came in last November on a high tide. His plurality of 1,390,137 votes was the largest of any governor in Illinois history. Many politicians think he carried Gerald Ford in Illinois, which the former President won by only 92,974 votes. In March, a popularity poll showed only 4 per cent disapproval.

One reason for Thompson's stratosperic popularity has been the contrast he provided to his abrasive predecessor, Democrat Daniel Walker. Where Walker battled the General Assembly, Thompson stroked it, appearing before the legislators to declare, "The war is over." The legislators responded by confirming every one of his cabinet appointments and by treating his $10 billion "austerity" budget with gentle care.

Thompson also wooed and largely won the press, going to the press room to drink beer with reporters and giving some of them his private phone number at the mansion with an invitation to call directly.

And Thompson continued to find favor with ordinary citizens by cultivating the relaxed, sometimes irreverent approach to politics that had marked his election campaign.

One day when Thompson was preciding over the Senate, the minister who was to read the opening prayer didn't show up and Thompson substituted for him, saying, "I can use the 50 bucks." Another time Thompson was presented a T-shirt at a high school and stripped to the waist to don it - causing a reporter to write that he had carried openness too far. Praising his running mate, Lt. Gov. David O'Neal, at a testimonial, he wove in an unsubtle reference to GOP Sen. Robert Dole's vice presidential campaign in 1976: "Most important of all, he never once called World War II a Democrat war."

Thompson also has made a fetish of racquetball, which he dredits with enabling him to reduce by 40 pounds. A bumper strip on the governor's car reads, "I'd rather be racquet balling."

All of this has served Thompson well with the media and the voters. But now there are complaints in Springfield that the personable Thompson has an unserious approach to the governorship. Legislators say to the governorship. Legislators say that "Big Jim," as nearly everyone calls him, is friendly but exerts little leadership. Sometimes his aides are unprepared to debate his bills; on rare occasions they have failed to show up at hearings. Some aides themselves have acknowledged that Thompson can be over-deliberative in his appointments and that he is reluctant to delegate.

Thompson's honeymoon with the press also may be on the skids. Reporters who thought the new governor was above reproach on issues of integrity were surprised to learn that Thampson had accepted Kentucky Derby tickets for himself and his wife Jayne and free airline trasportation to Louisville from the Chessie System, a railroad holding company that has subsidaries regulated by the state of Illinois.

Ironically, Thompson never saw the Derby itself. He had to leave Churchill Downs early to present a posthumous award to former Gov. Otto Kerner at the annual Lincoln Academy Ball in Peoria. As U.S. attorney, Thompson successfully prosecuted Kerner, then a federal judge, for taking part in a racetrack bribery scheme.

No one in Springfield thinks that Thompson is going to become another Kerner or that he is in danger of being bought by the Chessie System. Rather, those who know Thompson think his high opinion of himself makes him less sensitive to the appearances of improriety than he should be.

"Every man in public life, especially every man in political life, has to have the capacity to make ethical decisions for himself," Thompson says in explaining his Derby trip. "Otherwise, at some bottom line, you so totally destroy a man's own sense of self esteem, self worth and capability of self judgment that you very seriously impair the worthiness of his job."

Pressed by reporters on the issue, Thompson conceded a mistake and he apologized, both to the Chessie company for embarrassing its officials and to "supporters of mine whose perceptions held me to a higher standard." In the future, said Thompson, he would try harder.

Thompson displays a confidence, sometimes bordering on smugness, which may result from nearly always having things his way. He is the son of a well-to-do doctor and he was a big instant hit as an attorney and a prosecutor. The campaign was easy, too, and its outcome was never really in doubt once status quo Democrat Michael Howlett knocked off Walker in their primary.

This easy success - a deceptive easiness, say Thompson's friends, because he works hard and makes the difficult look easy - has continued in Springfield. The Democrats are divided by factionalism. Everyone, Democrat and Republican alike, knows that the voters want a balanced budget and no new taxes.

Within this context of limited expectations and his own limited goals, Thompson has done moderately well.

His cabinet appears to include many bright young men and women. His budget will be reasonably close to the $10 billion goal he set when he was elected. His modest government reorganization plans have been approved by both houses. His equally modest energy conservation package appears to be dead in the legislature. His ethics bills, which even the sponsors say were badly drafted, were killed in committee on a party-line vote.

While some Republicans privately criticize Thomason, some members of the Democratic opposition private praise him as a skillful politican. The best demonstration of this came when Thompson worked out an agreement with Chicago Mayor Michael Bilandic to build a freeway leg that will take out a number of homes occupied by blacks in Chicago. During the campaign, Thompson described a similar proposal by Howlett as "racist." Now he points out that it is part of a larger agreement that will free badly needed highway trust money to rebuild winter-battered roads.

Thompson appears to be building a kind of working coalition that will leave him free to travel after the legislative session and free to campaign, first for re-election in 1978 and then, if the opportunity arises, for the presidency in 1980.

Thompson was elected to a two-year term rather than the customary four years because a constitutional revision moved the gubernatorial elections away from presidential election years. It was widely assumed that this would be a political disadvantage because Thompson would face a hostile with little time to prove himself.

Like everything else, the supposed disadvantage has worked in Thompson's favor. The divided Democrats lack the time to develop a strong candidate, and Thompson has used the goslow mood of the electorate to nurture popularity with his own go-slow programs.

Only once, when he suggested that the legislature might have to raise gasoline taxes, has Thompson risked disfavor by courting a controversial issue. When its unpopularity became evident, he quickly dropped the idea.

Thompson says he enjoys being governor even though accomplishment is more difficult to measure and to achieve than in the prosecutor's office. But sometimes he seems to express a yen for the once and future campaigns.

"I find myself longing for the days of the campaign," he said during relaxed moment at the mansion. "As governor, you find that a lot of problems just are not soluble, or if they are, they're not soluble very quickly. It's a difficult job."