The Republican leadership of the new Senate has asked Alfonse D'Amato, the newly elected New York conservative, to be chairman of the District of Columbia appropriations subcommittee, which has line-item control over the city's budget.

D'Amato, who is vacationing in Europe, was unaware of his selection late yesterday. He had been asked to rank his top seven preferences for subcommittee assignments on the Appropriations Committee in a meeting of Senate Republicans at the Capitol last Thursday, "and D.C. wasn't even on the list," his transition chief, John Zagame, said last night.

Zagame said he would try to reach D'Amato by telephone in Belfast and break the news to his boss. "I wish I had some other news to give him," said Zagame, who said D'Amato had been hoping to be assigned to the transportation subcommittee or "'something else that has more to do with the affairs of New York state."

D'Amato's aide cautioned that it would be "premature to say that he has accepted it." But he added, "I won't deny he has been offered it."

For the last two years, the subcommittee has been chaired by Sen. Patrick J. Leahy (D-Vt.), who after a rocky start in his dealings with Mayor Marion Barry and city officials, came to be viewed as more of a friend than a foe of the city.

Leahy, for instance, chose not to impose line-item authority over the city's budget, even though he had that authority. For example, in a conference meeting with House members over differences in the two versions of the city's budget, Leahy sided with the city in a losing attempt to forestall the House's insistence that 200 additional police officers be hired. The city opposed the idea, saying it did not have the money to pay for the salaries.

It is not known what attitude D'Amato would hold on specific issues. Early in his campaign against liberal Democrat Rep. Elizabeth Holtzman, D'Amato stressed his opposition to abortion funding and the Equal Rights Amendment, although he later reversed his position on those politically sensitive issues.

The abortion question is a perennial battleground when the District's budget is up for review before Congress, and opposition from the Senate subcommittee chairman could threaten inclusion of money for abortions in the city's budget.

D'Amato, 43, campaigned on classic conservative issues, and rose from relative political obscurity after unseating the venerable Republican Sen. Jacob Javits, in a primary campaign in which D'Amato stressed the age and failing health of the 76-year-old incumbent.

A native of Brooklyn, D'Amato was a town supervisor in Hempstead, Nassau County. When his opponents ridiculed his look of statewide experience, D'Amato countered that the Long Island community has a population greater than seven states.

Sen. Charles McC. Mathias (R-Md.) had been in line to be chairman of both the Senate subcommittees that oversee District matters, but the liberal Marylander gave up his third-ranking spot on Appropriations for a chance to serve on the Foreign Relations Committee. Mathias will chair the Government Operations subcommittee on the District.

Leahy will remain on the Appropriations Committee, but will not stay on the District subcommittee. He said last week he would have stayed on as the ranking minority member if Mathias had accepted the chairmanship.

Zagame said that D'Amato will "look very carefully" at the D.C. subcommittee position because "he wants to serve responsibly, and somebody has to be the chairman."