Politics crowds in on population talks.

Wait a minute - wasn't this supposed to be a world conference on population? As delegations from 148 countries prepared to return home, delegates and reporters found themselves plunged into superpower rivalry, the Middle East, and the United States election campaign.

The first two issues were sharpened and magnified by the third.

* The United States wanted Recommendation 5 of the final document, calling for less arms spending and more money for economic growth, deleted as not directly connected to population. The Soviet Union insisted it be retained.

After days of argument, a compromise was reached late Friday. The US succeeded in having the recommendation removed from the body of the final document. But to please the USSR the recommendation was not dropped but retained in a ''limbo'' position between the document preamble and the main text.

* The US and Israel wanted Recommendation 34 deleted because it criticized occupation of territory by another (unnamed) country. Five Arab states tried to have ''Israeli occupation of Palestinian Arab land'' added. Israel raised the issue in Washington.

On the eve of the GOP convention in Dallas, and wanting to avoid upsetting Jewish voters, the White House remained firm. The Arabs withdrew their amendment , but at time of writing, no solution had been reached. A climactic vote was set for late Sunday afternoon to clear the way for the conference to end on schedule Monday or perhaps Tuesday.

The Middle East issue dominated the entire session on Sunday amid UN and delegates' concern that the US might go as far as walking out if it lost the vote.

The original language of the recommendation drew no US or Israeli objection when it appeared at a preparatory conference earlier this year. It was only later that Washington and Tel Aviv began to express concern. Reprimands have been issued inside the State Department.

* US presidential politics was an issue within many delegations. The third world saw it as at least partly responsible for a change in US family-planning policies and for the US emphasis here on free-enterprise incentives as the best way to promote economic growth.

Some UN officials, with hindsight, thought it unfortunate that the second world population conference should have been held in the same month as the GOP political convention in Dallas.

The date was fixed because the first world conference was held in Bucharest in August 1974. A better date might have been in January before the primary election season. Anything later than August would have conflicted with the United Nations General Assembly session in New York.

Nonetheless, beneath the tumult, progress was reported on the population front as well.

The White House, under sharp pre-convention pressure from pro-family-planning members of House and Senate, finally decided Friday to release $19 million in contributions to the UN Fund for Population Activities (UNFPA). The money was handed over here Saturday after being impounded by the Reagan administration for two months while US aid policies were being tightened.

The White House was also pushed into action by the arrival here Saturday of six congressmen, including one Republican, whose purpose was to criticize tightened US policy on family-planning aid and to demand that Congress be consulted at once. A flurry of telephone calls from Washington galvanized negotiations which had taken place over five days between the US delegation and UNFPA executive director Raphael Salas.

The final deal was worked out in a hotel room early Saturday. Mr. Salas released a letter actually written earlier in July to US Ambassador to the UN Jeane Kirkpatrick, repeating previous assurances that UNFPA ''does not support abortion as a method of family planning nor does it sanction - nor has it ever sanctioned - coercion in the implementation of family-planning programs.''

The UNFPA did not change any of its activities but simply gave what was the fifth in a series of assurances, slightly reworded in one part to meet new US policies. Those controversial US policies first appeared at the end of May in a White House policy document for the conference which would have in effect cut off US government-to-government aid to any other country that permitted abortion.

This switch caused an outcry in Congress and among private family-planning groups. The policy was eventually softened to restrict aid only to private organizations rather than governments - but remains a center of controversy.

For two months the UNFPA sought release of the $19 million, which was the second half of the US fiscal 1984 contribution. The first half had been released many months ago. The US is the biggest single contributor to the UNFPA budget. US delegation leader James Buckley announced its hand-over Saturday here together with Agency of International Development chief Peter M. McPherson.

A few minutes later in the same Mexico City hotel, Rep. James Scheuer (D) of New York and five other members of the House of Representatives were criticizing new US family-planning policies. They comprised a US congressional delegation to a parliamentarians' meeting on population after the main conference.

Rep. Patricia Schroeder (D) of Colorado spoke for all when she said the administration had been ''shortsighted'' not to have included anyone from Congress on the official US conference delegation.

They warned the Reagan administration that they objected to the US having one law at home (health centers using nongovernment funds for abortion-related activities can still get federal money for other purposes) and a different one abroad (any private organization using any funds whatever in connection with abortion can receive no US funds).

Unless Congress is consulted, they said, they may have to order a change in ''continuing resolutions'' authorizing government expenditures after Sept. 30.

Meanwhile, Mexican anti-abortion groups staged a protest march against the population conference Saturday. About 6,000 people marched to the Basilica of Guadeloupe for an outdoor mass - far fewer than organizers had hoped for.