It's the unfinished agenda that beckons the President toward second-term run

Why is Ronald Reagan running for a second term? ''The job is undone,'' says Sen. Paul Laxalt (R) of Nevada, the President's closest confidant in Congress and general chairman of the Republican Party. While the basic domestic agenda has been set, Senator Laxalt indicates, Mr. Reagan still needs to see through a whole range of programs.

At the top of the list Laxalt puts budget and fiscal policy, which would include resolving the deficit problem and bringing the budget under control. He also cites further regulatory reform and further steps to make government ''work better.''

''We've only scratched the surface on crime,'' he adds, ''and there's a lot to do on drug abuse.''

Talk of a second term assumes Reagan is running. Despite reports that the President intends to announce his candidacy soon after his State of the Union message in January, doubts persist in some quarters that Reagan has made up his mind. But Laxalt, who in October was authorized by the President to set up the Reagan-Bush reelection committee, says he would not have taken the job of committee chairman if there were any doubt in his own mind about Reagan's intentions.

''He has never said to me, 'Paul, I'm going to run.' But all conversations over the past year have been in the context of a candidacy,'' Laxalt told the Monitor in an interview earlier this week. Reagan has not been agonizing. ''I assume he has made up his mind.''

On the foreign policy front, much remains to be done because everything is in a ''state of transition,'' the senator says. He sees no major arms-control agreement with the Soviet Union until the second term.

Laxalt, a man who holds considerable influence with the President and in the Republican Party, joins other leading GOP strategists in voicing concern about developments in the Middle East in a political as well as foreign policy context. ''Lebanon poses a potential vulnerablity,'' Laxalt says. ''It depends on where we are eight or nine months from now. If the situation is unchanged, this will be a serious political problem. The American people will be impatient. They would not like having the marines there.''

Central America is viewed as another unpredictable situation for the President. ''It's not as important as the Middle East,'' says Laxalt, ''but it presents another vulnerability.''

Laxalt insists, however, as do White House officials, that the President is not being guided by political considerations in the conduct of foreign policy. In East-West relations, the senator says, ''he's letting the political chips fall where they may. Politics is not a factor.''

What does he regard as the President's foreign policy achievements? ''There's been no major war,'' the senator comments, ''and he's making satisfactory progress in a number of areas.''

The massive budget deficits, which Democratic candidates are vigorously attacking as a Reagan liability, are not a political problem at the moment, Laxalt says. ''The deficit becomes a problem only if interest rates go up,'' he adds, ''and there is no sign of that.''

The GOP strategy in the 1984 campaign, Laxalt says, will be to have Reagan seen by the public as doing a good job as president. ''We won't confine him to the Rose Garden,'' he says. ''He will campaign aggressively, but the principal strategy will be to have him be the President.''

Reagan's major asset is seen to be the quality of leadership he has provided. In Laxalt's view, Reagan has done away with the ''myth'' that the presidency is such a complicated job that no one can handle it. In Laxalt's words: ''He has provided style and personal dignity, and in handling states and formal events, style is important.''

Above all, says the key GOP strategist, Reagan has fundamentally changed the debate on the economy and the role of government. It is now realized, even on Capitol Hill, that government is not a bottomless well and that there has to be restraint on spending, he says.

Asked whether the President had made any mistakes in his first term, Senator Laxalt replied, ''No prominent ones. . . . He's done a remarkable job.''