President Reagan and Walter F. Mondale spent yesterday preparing for their nationally televised debate tonight in Louisville, but the sponsors of the debate were having trouble getting the two camps to agree on a panel of four reporters to ask the questions.

The sponsors, the League of Women Voters, said the two political organizations had finally agreed on a panel of four reporters after rejecting more than 100 names, but one pulled out early last night. League President Dorothy S. Ridings said the league would proceed with three panelists and the moderator, Barbara Walters.

Gerald Boyd of The New York Times, the last reporter chosen, abruptly announced his withdrawal shortly after being picked. "I really don't think it leads to the most worthwhile debate if you've got a situation where both camps are deciding who should ask questions," Boyd said.

The three panelists who have agreed to serve are Fred Barnes of the Baltimore Sun, Diane Sawyer of CBS News and Jim Wieghart of Scripps-Howard.

Tonight's debate, the first of two scheduled encounters between Reagan and Mondale, offers the Democratic nominee his best opportunity in the campaign to begin to close the gap with the president.

The debate is scheduled to begin at 9 p.m. EDT.

Reagan spent the day at the presidential retreat at Camp David, interrupting debate preparations to deliver his weekly five-minute radio broadcast at noon.

Mondale spent most of the day at home getting ready for the debate, but took a two-mile walk with his family and dogs along the C&O canal here in the early afternoon sunshine.

Both will fly to Louisville this afternoon.

Ridings said the league had given the names of more than 100 reporters to both campaigns so each could pick a panel of four, but all but Barnes, Sawyer and Wieghart had been vetoed by one camp or the other. Later she announced agreement on Boyd only to have him withdraw.

Outlining the problem, Ridings said the league first gave the two camps the names of 12 "first-rate journalists" and, when those were exhausted, gave them 100 more.

She said the vetoes "were almost equally divided between the two campaigns . . . . You can imagine our dismay . . . . " She said there were no such vetoes in 1976 and many fewer in 1980 when similar selection procedures were used. She said the league would submit no more names.

In his radio address, Reagan denounced Mondale's proposed plan to interdict the flow of illegal drugs into the country, estimated to be worth $100 billion annually, saying he has already implemented all of the measures Mondale proposes. Mondale suggested a federal drug czar to coordinate the fight, the use of the military to stop shipments and denial of foreign aid to uncooperative nations.

Reagan said all of that has been done under the direction of Vice President Bush and Attorney General William French Smith.

Mondale said he looked forward to the opportunity to spell out his differences with Reagan. "There are deep issues here about America . . . , where we want to go that are at stake here," he said. "And Americans have this chance every four years to decide what they believe in. Never have those issues been clearer than in 1984."

With 30 days remaining until Election Day, Mondale trails Reagan by 18 points, 37 percent to 55 percent, in the latest Washington Post-ABC News poll and by 17 points in the latest Gallup Poll. Thus, there has been essentially no movement since late June, when the Gallup Poll showed Mondale trailing by 19 points, 37 percent to Reagan's 56 percent.

Mondale and his advisers see the debates as opportunities to finally display his mastery of the issues in direct comparison with Reagan.

"Ninety minutes with Reagan will do us an enormous amount of good, more than 20-second cuts, because it's harder for him to slip off specific questions," said Robert G. Beckel, Mondale's campaign manager.

Debates in the past have benefited other trailing candidates, but none was behind by a margin such as Mondale's.

In 1960, John F. Kennedy took a 3-point lead over Richard M. Nixon after their first debate but was trailing by just 1 point when it began. In 1980, Reagan took the lead and won the election after his debate with Jimmy Carter but had trailed by just 6 points before the debate.

Part of Mondale's problem is that he and Reagan have been figuratively arguing apples and oranges in this race so far.

Late last year, Richard B. Wirthlin, Reagan's pollster, predicted that this presidential campaign would produce "a classic conflict on the issues," among which he included arms control, U.S.-Soviet relations, defense preparedness, the Middle East, Latin America, human rights, civil rights, education, taxes, the budget deficit, welfare, the environment and others.

Those are the issues Mondale has tried to raise at every turn, but it has resembled the sound of one hand clapping. Reagan has ignored him on everything but the bombing of the U.S. Embassy annex in east Beirut two weeks ago and the indictment of Labor Secretary Raymond J. Donovan.

Reagan, not Mondale, has framed the issues of the race. They are: Reagan's first term, the economic recovery, leadership, optimism versus pessimism and the future versus the past. Two Reagan campaign lines sum them up.

"We see every day as the Fourth of July," he said at one campaign stop. "The other side sees every day as April 15." On another occasion, he celebrated his party of "high tech" versus the party of "high taxes."

In addition to the embassy bombing and the Donovan indictment, Mondale has also taken Reagan to task for not exercising leadership on arms control and on the question of raising taxes to help reduce budget deficits, which Mondale contends are a threat to the economy.

But Reagan demonstrated the power of the incumbency by inviting Soviet Foreign Minister Andrei A. Gromyko to the White House and making a conciliatory speech to the United Nations that one Democrat grumbled "could have been written by George McGovern."

The tax and deficit issue also appears to have rebounded on Mondale. Many voters' response is to view Mondale as eager to raise their taxes and to believe that Reagan would do it only as a last resort. Few appear to have focused on Mondale's message that the budget deficits will increase interest rates and help abort the economic recovery.

The Democrats had counted heavily on registering new voters, particularly blacks in the southern states, which Reagan won narrowly in 1980, to help offset the advantage of his incumbency. The latest Post-ABC poll, however, indicates that while a larger proportion of the new voters favor the Democrats than does the entire national sample, Republicans still have a substantial 14-point advantage, 54 percent to 40 percent, among those registered since 1980.

As a result, many Democrats are giving top priority to electing U.S. senators and House members. Some Republicans are predicting that Reagan's anticipated coattail effect could help reelect such seriously threatened Republican senators as Jesse Helms of North Carolina, Roger W. Jepsen of Iowa and Charles H. Percy of Illinois and give the party a net gain of as many as 40 House seats. The Democrats contend that their candidates have put enough distance between themselves and the national ticket to prevent this, however.

If Mondale is able to turn this race around, it will replace Harry S Truman's 1948 victory as the standard by which upsets are measured. If he doesn't, the evidence is that Reagan really won this presidential election in 1983 -- when the economic recovery got under way.

In January 1983, Reagan's favorable job approval rating had sagged to 38 percent, unemployment was almost 11 percent, there was squabbling within the administration and in the Republican Party and there was a serious question whether he would run for reelection. The Gross National Product had shrunk by 2.1 percent in 1982 as a result of the recession.

The economic recovery began at the end of 1982, however, and by the end of 1983 GNP had increased by 3.7 percent (and is projected to grow by a whopping 7.3 percent in 1984) and unemployment was down to 7.9 percent (it is 7.4 percent now, about what it was when Reagan took office). According to a Newsweek poll, Reagan's job approval rating last Jan. 1 had risen to 56 percent, the highest since Dwight D. Eisenhower's in 1956 and the same as Reagan's projected vote in the current polls.

Two major factors in the recovery were the loosening of the money supply and the stimulus of Reagan's tax cuts -- ironically, a Keynesian solution that the Democrats have used for half a century, and a major factor in the budget deficits Mondale so deplores.

The same Newsweek poll showed that the number of respondents who strongly disapproved of Reagan dropped overall, from 35 percent to 20 percent. More ominous, from Mondale's point of view, was that the number of Democrats who strongly disapproved of Reagan plummeted more dramatically -- from 58 percent to 30 percent.

A nationwide Post-ABC poll last January indicated that the median annual income of the respondents was $20,000: about half reported annual incomes of less than $20,000, half reported annual incomes of more than $20,000.

Overall, Reagan narrowly led Mondale in the poll, 48 percent to 45 percent. Of those under $20,000 last January, only 36 percent indicated they would vote for Reagan while 59 percent preferred Mondale. Of those at more than $20,000, however, 58 percent preferred Reagan with 37 percent for Mondale