The Senate Judiciary Committee, preparing to deal this week with the politically sensitive issue of extending the 1965 Voting Rights Act, has been the target of heavy, last-minute lobbying by civil rights activists and the Reagan administration.

The controversy centers on what standard of proof should be used to determine whether minorities have been illegally excluded from voting.

The administration wants a simple 10-year extension of the act, retaining the Supreme Court finding that state and local officials violate the law only if they intentionally plan to discriminate.

But 65 senators are cosponsoring a much more liberal version of the bill--identical to one passed by the House, 389 to 24--providing that a violation occurs whenever local election laws have the practical effect of discriminating against blacks or other minorities. Justice Department officials stepped up their lobbying in the past week, claiming that a recent court decision in Mobile, Ala., shows that the intent test can be met easily. In a letter to members of the Judiciary Committee, Robert A. McConnell, assistant attorney general for legislative affairs, said, "It is now clear beyond doubt that the asserted reason for changing the law --that the existing intent test is too difficult--has no basis in fact."

But civil rights lawyers who favor an effects test insist it is almost impossible to prove intent, especially in cases where the elected officials responsible for the laws have been dead for years.

The Mobile decision is symbolic because it is the case the Supreme Court used in 1980 to set the precedent requiring the intent standard. Until then, lower courts had been divided on the issue.

The Supreme Court sent the Mobile case back to the lower court, where blacks had originally won, saying intentional discrimination had not been proved. Since then, lawyers for blacks have been working overtime collecting evidence that Mobile's election system, which dates to the 19th century, intentionally discriminated against blacks.

The Leadership Conference on Civil Rights, an umbrella group of civil rights organizations, has told the committee that although it was possible to prove intent in this case, it took more than 6,000 hours of lawyers' time and 4,000 hours of time on the part of expert witnesses and paralegals.

In making his decision, Judge Virgil Pittman found it necessary to study Mobile's municipal history back to 1814.

The debate on the standard of proof has produced virtually no agreement between the two sides.

Administration officials have repeatedly insisted that an effects standard would lead to proportional representation by race in cities and towns across America. They say that most of the country's municipal election systems likely would be challenged in court.

Proponents of the effects test say that is not true. They have included language in their bill specifying that it should not be interpreted by the courts to call for proportional representation.

With the two sides so polarized, Sen. Robert J. Dole (R-Kan.) has begun an effort to promote a compromise to avoid a deadlock, but few members on either side seem to be buying it.

Of the 18 committee members, nine are co-sponsors of the bill calling for the effects test, seven support the administration position, and two--Dole and Sen. Howell Heflin (D-Ala.)--are publicly uncommitted.

Dole's plan would require plaintiffs to prove that the political process is not equally open to minorities. His plan would set up a list of discriminatory factors a court could use in finding a violation. These include a history of official discrimination, racially polarized voting patterns, under-representation of minorities in elected positions and "invidious discrimination in such areas as education, employment, economics, health and politics."

After some refining, he is expected to offer his plan as a substitute when the issue comes before the committee tomorrow.

But an aide to a Judiciary Committee conservative said, "This is just an effects test disguised with a lot of words. It would guarantee minorities not only the right to vote, but the right to win. This is real radical for Bob Dole."

And one source on the other side said, "We've got 65 votes. That's two-thirds of the Senate. What do we want to compromise for?"