Who was that shaking hands with Los Angeles Dodger superstar Fernando Valenzuela on the front page of a recent campaign mailing?

That former John Birch Society member, a gringo's gringo, Rep. John H. Rousselot (R-Calif.).

Who was that who just joined the Hispanic Congressional Caucus, so quietly its leaders never knew until it was too late?

The same Rep. Rousselot, despite his Franco-Irish ancestry.

Who spent $1,435 in campaign funds for his wife's Spanish lessons? You guessed it.

There he was, the man who today represents the upper-crust matrons of San Marino, resplendent in a south-of-the-border shirt, riding in the Mexican Independence Day parade through East Los Angeles.

"Viva Mexico!" he cried.

His 26th Congressional District shot out from under him by redistricting, Rousselot is pursuing an energetic ethnic game plan for winning a seat in the new 30th District, contested by fellow incumbent -- and bona fide Hispanic -- Rep. Matthew G. (Marty) Martinez (D-Calif.).

What does Martinez think of Rousselot's parade appearance?

"Gee, isn't that wonderful?" says Martinez. "He's learned a Mexican word."

The story of John and Marty and the citizens of flat and smoggy East Los Angeles begins with the bizarre California redistricting plan formulated last year by Democratic Rep. Phillip Burton. It was designed in part to usher Burton's old enemy Rousselot out of Congress.

Rousselot could easily have moved into the nearby, affluent, relentlessly Republican district of Rep. Carlos J. Moorhead. The energetic and resourceful Rousselot would have had a good chance of beating the diffident Moorhead, but Moorhead is an old friend.

Rep. George E. Danielson, who represented the old 30th District, resigned to become a judge. Martinez, 53, a former Monterey Park mayor and state assemblyman, won a special election in June to serve out the remainder of Danielson's term.

But in the redistricting, Burton cut the Latino population of the 30th District from about 62 to 53 percent, and Rousselot saw his chance.

By his reckoning, Latinos made up only about one-third of the district's registered voters and at least 20 percent of them were Republicans. He also knew they are more sympathetic than other minorities to some Republican issues, such as opposition to abortion.

Martinez contends that voters are more concerned with a representative's experience than his last name.

"I'm not a Hispanic candidate," he says. "I'm an American candidate."

The district's 64 percent Democratic registration obviously favors him and he benefits from his ties to a rising new political machine headed by Assemblyman Howard Berman, also on his way to Congress.

Martinez regularly derides the economic policies of President Reagan and paints Reagan and Rousselot as close allies. He refers to Rousselot as "J.R." (of the "Dallas" television show), "a despicable carpetbagger from the rich money interests" who, he says, "has voted constantly to gut Social Security."

Rousselot characteristically does not back away from the Social Security issue, as a more prudent politician might. However, he spends a lot of time explaining to senior citizen groups why he thinks the rate of increase in cost-of-living benefits should be reduced so that it does not exceed the inflation rate and bankrupt the system.

"That seems fair," said one white-haired Alhambra woman to another after one such friendly Rousselot economics lecture.

Martinez plans to spend about $200,000 in the campaign; Rousselot is budgeting about $450,000.

Martinez is one of only two California congressmen of Latino heritage, despite the state's approximately 20 percent Latino population. But he thinks this time people will finally remember "the Anglo politicians who used to come into the district, yell 'Viva La Raza,' and when they got the vote didn't do anything for anyone in the district, Hispanic or otherwise."