White House chief of staff Donald T. Regan has selected new presidential assistants for politics and legislative affairs, part of stepped-up preparations for a major policy "offensive" on spending and tax issues by President Reagan in the fall.

Officials said Regan has decided to name Mitchell E. Daniels Jr. to succeed Edward J. Rollins as White House political director. Daniels is currently a deputy to Rollins and was previously executive director of the Senate Republican campaign committee.

Regan has also decided to give M.B. Oglesby expanded duties for congressional liaison, which he now shares with strategist Max L. Friedersdorf, who is leaving the White House later this year.

The appointments are to be announced Monday as part of what one senior official calls an "aggressive" planning effort to cope with a crowded and critical period when Reagan returns from his California vacation and recuperation from cancer surgery.

The new planning comes as officials acknowledge that Reagan has experienced major political difficulties during the first six months of his second term, a period that was supposed to be a "window of opportunity" for advancing his program following Reagan's landslide reelection.

After a series of setbacks and squabbles with fellow Republicans in Congress over the budget and taxes this year, chief of staff Regan is said to be determined that the president regain control over the agenda, and prevail on key issues in the three-month stretch between Labor Day and Thanksgiving.

"They've got to hit the deck running in September," an informed official said.

Two other staff shifts are expected, officials said. Public liaison director Linda Chavez is being moved after several months of delay to the prestigious West Wing from the Old Executive Office Building, and will carry out a significant part of the autumn political blitz. William Henkel, director of presidential advance, is also being given greater responsibilities as part of the effort, officials said.

The president announced last Monday that he would undertake a "major fall offensive" by "pulling out all the stops for passage of tax reform" and confronting Congress on spending bills with "my veto pen hovering over every line."

But White House officials have been worried in recent weeks that Reagan could run afoul of a cramped congressional agenda. Regan convened a series of staff meetings this week to plan the fall offensive; more sessions are expected in Santa Barbara next week.

White House officials have been predicting recently that Reagan will confront Congress with vetoes over spending bills this fall. But Reagan has in the past rarely vetoed appropriations bills, largely because they were negotiated to his satisfaction by then-Office of Management and Budget Director David A. Stockman.

A "veto strategy" requires a different approach, officials said, and the White House is studying the prospects for vetoes of specific bills that can be sustained this fall.

Presidential spokesman Larry Speakes said yesterday that the farm bill pending on Capitol Hill "is a serious contender to be classified as a budget buster" because the cost has "doubled or tripled since Congress started working on it."

"I can assure you that the president will do his part to hold down spending, and if that means veto, then he will veto," Speakes added.

At the same time, Reagan is facing a congressional battle over raising the federal debt ceiling to $2 trillion. He also will be in the midst of congressional debates on trade and the farm bill. The president also wants to deliver a series of speeches to drum up support for his tax plan, passage of which will require bipartisan support.

Meanwhile, the president also will be preparing for a meeting with Soviet Foreign Minister Eduard Shevardnadze in September and with Soviet leader Mikhail Gorbachev in November, both of which will demand increasingly large amounts of his time.

The appointments of Daniels and Oglesby are being made now, officials said, to allow them to join in planning the fall offensive, although they may not assume new duties until October. Others playing a major role in the planning are communications director Patrick J. Buchanan and Regan deputy Dennis Thomas.

Rollins, who is leaving the White House Oct. 1 to enter the political consulting business, said yesterday that he recommended Daniels because "he has good antennae and good political judgment. He'll be less controversial than I am."

Oglesby will be taking up a role he performed in 1984 as chief congressional liaison. This year, the duties were shared with Friedersdorf, who held the added title of legislative strategy coordinator. Friedersdorf is expected to be named to a diplomatic post in Bermuda this autumn.