West Germany's major opposition party, the Christian Democrats, today bypassed their party leader, Helmut Kohl, and proposed state governor Ernst Albrecht as their candidate to challenge Chancellor Helmut Schmidt in next year's federal elections.

The decision by the party's Executive Committee to back the Maerican-educated Albrecht, 48, marks a major turning point in opposition politics here and could reopen a split between the Christian Democrats and their Bavarian sister party, the Christian Social Union.

Last week Franz Josef Strauss, 63, the colorful and controversial leader of the Christian Social Union, grabbed the initiative from the larger Christian Democratic Party by announcing that he would be a candidate for chancellor.

Thus the two conservative parties, which have formed a parliamentary alliance for 30 years, must decide which of these politicians will be the candidate in the fall of 1980 or if the parties will split and run separate candidates.

In October 1976, with Kohl as the challenger, the conservatives came within 1 percent of the votes needed to oust Schmidt from the chancellery.

Since then, Kohl, 49, has turned out to be a disappointing party leader in parliament while Schmidt, 60, has increased substantially his power and prestige to the point where he is now perhaps the most powerful chancellor since World War II. Thus, whoever challenges Schmidt will be fighting long odds.

Kohl said today that his fellow moderate Albrecht, the governor of Lower Saxony, reflected the moderate road in conservative politics in a "credible and convincing way."

In effect, the battle shaping up among the conservatives will determine whether there is a move away from the policies of Kohl and Albrecht to the right-wing appeal of Strauss.

Strauss, who is governor of the state of Bavaria, has sought to move into the national power vacuum of the conservatives created by Kohl's weak leadership.

Although Schmidt's coalition is generally just to the left of center in West German politics, the chancellor is more conservative than his party and thus able to satisfy a wide range of voters.

Schmidt is known to feel that if Strauss, whose image and views have always been portrayed as ultraconservative, becomes the candidate, it will not only alarm many West Germans but also feed anxieties among West Germany's neighbors.

Albrecht, who studied at Cornell University, entered active politics eight years ago and became chief of his state in 1976. He increased his majority at home in elections last year and in recent months he won praise for outmaneuvering Schmidt on a crucial nuclear energy question.

Albrecht rejected a proposal that would have placed a nuclear reprocessing plant in his state and then blamed the rejection on lack of support by Schmidt's own party.

Kohl said today that while he will step aside as a candidate, he intends to retain his posts as party chairman and parliamentary leader.