President Reagan remains undecided on the timing of a major lobbying effort intended to secure congressional approval of $14 million in aid for the "contra" rebels fighting the leftist Sandinista government of Nicaragua, administration officials said yesterday.

"There is no question about the president's commitment; it's entirely a matter of what's the best timing," an official said.

The official said that Reagan has received conflicting recommendations on whether to push for the funding in April or to wait until after he returns from an 11-day trip to Europe on May 10. The key factors in the decision are expected to be the head count in the House, where the administration believes it is at least 20 votes short of approval, and the military situation in Nicaragua.

If Reagan decides to push for immediate approval of the funding, he probably will make a nationally televised speech later this month and possibly address a rally in the Orange Bowl in Miami between April 25 and April 28, officials said. Otherwise, he may wait until as late as June to make an all-out push for the funds.

Reagan has been emphasizing the issue, however, in speeches and interviews for the past month.

Meanwhile, Rep. Lee Hamilton (D-Ind.), chairman of the House Permanent Select Committee on Intelligence, confirmed that he will lead a bipartisan delegation of committee members on a four-day swing through Central America next week.

Hamilton said the visit, scheduled to begin April 12 and end April 15, is not a response to a recent invitation from President Daniel Ortega of Nicaragua for a bipartisan group to inspect his armed forces.

However, the group of eight or nine members plus staff aides will consider military matters among other things and will speak with Ortega and national leaders in Guatemala, Honduras, Costa Rica, Nicaragua and possibly El Salvador, Hamilton said.