In a sense, Yusef Jackson and Jimmy Kemp are as different as two teen-agers can be. One is black, the other white. One grew up in the Midwest but attends boarding school in the East. The other grew up in suburban Maryland and goes to public school there. One is a Democrat, a liberal under almost any definition of the word. The other is a Republican and a self-described conservative.

But the similarities between the two are more striking than the differences. Both are very good high school football players, each the younger brother of a star player who played under the same coach at the same school. Each is a fourth child and a second son and has had to deal with expectations thrust upon them because of the success of their older brothers. Each is from a close family.

And, each has a father who has ambitions to become president.

"When people label me 'Jesse Jackson's son', I don't worry about it," Yusef Jackson said recently. "All they're doing is telling the truth. I play football because I like it. My father is like any other father when it comes to my football." He paused and smiled. "He's a critic."

So is Jack Kemp. He is as conservative and Republican as Jesse Jackson is liberal and Democratic. Jesse Jackson was a star linebacker in college but Kemp went one step further, starring throughout the 1960s for the Buffalo Bills. If you listen to his son tell the story, it was football that got Jack Kemp into Congress 17 years and nine elections ago.

"My dad was in a very close race that year," Jimmy Kemp said, laughing because he was telling a story his father enjoys telling on himself. "A television reporter asked him one day what he would do if he lost the election. My dad had two years left on his contract so he just told him he'd probably go back and play quarterback for the Bills.

"That night the guy, his name was Irv Weinstein, went on the news and said, 'Today, Jack Kemp issued a warning to all Buffalo football fans saying that if he is not elected to Congress, he will return to play quarterback for the Bills next season.' "

Jimmy Kemp smiled as he delivered the punch line. "My dad won by 1 percent of the vote."

The kid has the makings of a great politician.

First, though, there is football. Jimmy Kemp is 12 years younger than his brother Jeff, who played at Churchill High School, then at Dartmouth and is now with the Seattle Seahawks after stints with the Los Angeles Rams and San Francisco 49ers.

Jeff Kemp's eight-year career in the NFL is nothing short of amazing to Fred Shepherd, the man who has coached both Kemps in high school.

"Jimmy is so much better at this stage of his life than Jeff was it's unbelievable," Shepherd said. "Jeff didn't even start until he was a senior. If you had asked me then if he would ever play in the pros, I would have said, 'Never, never, never.' We've had a lot of kids play here over the years who were better high school quarterbacks than Jeff Kemp. But you can't measure what's inside a youngster."

Genetics may have a lot to do with it. Jeff and Jimmy Kemp were never pushed to play football or to be quarterbacks, but they were never discouraged to look in that direction, either.

"When I was young, Sunday was always football day in our house," Jimmy Kemp said. "When Jeff was in high school, he and my dad would talk about every game he played in detail, go through mistakes and things. Usually, in the afternoon I would go out in the yard by myself and play. I was always Roger Staubach -- he was my big hero back then. That was when I was like 4 or 5. When I got old enough to play, I was a halfback and a quarterback for one year but after that I was always a quarterback. I could throw pretty well for my age."

Kemp can still throw well for his age. Although he is only a junior, he is already as big as his brother at 6 feet and 173 pounds. The two work out in the summer, often playing games on the beach to see who has a stronger, more accurate arm.

"It's funny, because I've always been a lot more aware of the fact that Jeff was a quarterback than I was aware of my dad being a quarterback," the youngest Kemp said. "By the time I was born, my father had retired and was already in Congress so that's the way I've thought of him. But ever since I can remember, Jeff's been a quarterback."

Now that Jimmy is a quarterback, too, he has two critics in the family. This being the age of the VCR, he and his father go through a tape of his game each Saturday. Often, he and his brother talk the game through by phone after that.

"Jimmy is lucky because he grew up with Jeff as a role model," Jack Kemp said. "He's more of a natural quarterback than Jeff, but Jeff really worked to make himself into a quarterback. They're both tenacious, though, and that makes them good football players."Attendant Pressures

Naturally, there are pressures that come from being the son of a celebrity, more specifically the son of an often controversial politician who is running for president. Jimmy Kemp does not see himself following in his father's post-football footsteps.

"Some of the things about politics I've really enjoyed," he said. "I've met the last three presidents; not many kids my age can say that. I do like going to a place like Iowa and meeting people because they're so different from people here.

"But I really don't think I'd like fund-raising. It's a little bit too much like begging for me. And the fakeness bothers me, too. There's a point where you just want to be real with people and in politics you can't always do that."

There is also the problem of picking up a newspaper or turning on a television set and reading or hearing things about your father you just don't believe. Several weeks ago, as Jimmy Kemp sat reading a story about his father that was less than flattering in its description of his chances, he started to get upset.

"We were sitting in the living room and I was reading it and my dad saw me getting upset," he said. "He said to me, 'Jimmy, just put it down. Stories like that are part of politics. You don't have to read it.' It's hard, though."

Both sons seem to be able to handle their father. When Jeff was at Dartmouth, his father once came down at halftime of a Harvard game and yelled, "Jeff, the quick-out is wide open."

"I know, Dad," Jeff yelled back, "go back and sit down."

Last week, as Churchill played Wootton, Jack Kemp walked behind the bench and began to offer Jimmy some advice. "Dad," Jimmy Kemp said, "I can handle things, don't worry."

One thing will change in the Kemps' life for certain next year: Jack Kemp won't be back in Congress. Jimmy Kemp never has known his father as anything but a congressman. "Our lives are going to change next year no matter what happens," he said. "If Dad doesn't win, I'm not sure what he'll do. He once told me that if he really wanted power, the job he would go after would be commissioner of the National Football League. Either way, though, politics will always be a part of our lives. It isn't apt to change after all these years."

In the meantime, Jimmy Kemp has other things to worry about. Like Shepherd's concern with his lack of time in the weight room; like honors English and history; like whether Stanford will recruit him and give him a chance to follow in the footsteps of his current hero, John Elway.

"All in all," Jimmy Kemp said, "there aren't too many things to worry about when you're in high school. I'm just having a good time."

Unlike Jimmy Kemp, Yusef Jackson does not radiate political savvy when he talks about himself. He seems quieter -- confident, but reserved. He is the son of a man who can fill a room with huge emotions and yet he keeps most of his emotions to himself.

"The politician in the family is little Jesse," said Dick Allanson, the St. Albans coach who coached Jesse Jr. and now coaches Yusef, a 6-2, 215-pound senior who plays fullback and linebacker. "If you spent five minutes with little Jesse, you would know he was Jesse Jackson's son. He was bright, brash, articulate and political. Yusef keeps more to himself."

Except on a football field. Yusef Jackson loves to hit. "For some people," he said, "the need to hit is as natural as the need to breathe."

No one loves that approach to football more than Jesse Jackson Sr. "That's the way I want him to play," Jackson said. "I got on him a couple weeks ago because he was so eager to get into the backfield that he was reacting too quickly. But most of the time when he makes a good hit or gets off a good block, I'm jumping up and down and hollering as loud as anyone out there.

"Last year, he took off on a 93-yard run and I got so excited I beat him to the end zone. He may well be the best player in our family. Jesse got more interested in politics than football when he was in college. I think Yusef will be a very good college player."

So do the college recruiters. Many top schools are interested in him, partly because he is a very young high school senior (he just turned 17), partly because he is a solid student (more disciplined than his brother, according to his coach and his father) and mostly because of that ability to hit.

Ironically, when Yusef Jackson first arrived at St. Albans as a boarding school student from Chicago, most people thought he would be a small, quick halfback like his older brother. "I was about 5-8 and 140 pounds and I couldn't lift 135," he said. "Everyone thought I was a scatback like Jesse, but I wasn't fast. It led to a pitiful freshman year."

But the youngest son kept growing and is now bigger than his brother, who played for two years at North Carolina A&T before giving up football.

"I think my brother will end up in politics, no doubt," Yusef said. "But not me. Right now, I want to go as far as football will take me and then get into some kind of business later on."

Football is his main business now, though. Like the Kemps, the Jacksons talk through games when they're over. "Deep down, my dad is a coach," Yusef Jackson said. "He's a tough critic. He gets on me for tackling too high or being overaggressive."

Jesse Jackson says he loves nothing more than watching his sons play football. "When I first ran for president in '84, I would go to Jesse's games and when he would take off the crowd would yell, 'Run, Jesse, Run,' " the father said. "I liked to think it was for me -- but it wasn't. Now, I make Yusef's games whenever I can."

Yusef Jackson says he hasn't thought about the problems that would go with having a president for a father. Jesse Jackson says it won't be a problem if it happens. "I'll still go to the games," he said. "I'm not going to miss seeing my son play. Security won't be a problem. I've taken Secret Service with me to football games in the past. They like football, too."

Of course, if he were president, Jesse Jackson might have to give up dashes to the end zone to greet his son. But that is a long way off.

"I'm very proud of my father," Yusef Jackson said, "no matter what happens. He's always been a tough disciplinarian, but he's also been very encouraging. I don't mind it when people point and say I'm Jesse Jackson's son. I haven't established my own separate identity yet. But I will. Around here, I'm just one of the guys. And that's just fine with me."