As Sen. Robert J. Dole (Kan.) prepares for Monday's formal announcement of his candidacy for the Republican presidential nomination, key supporters and strategists are increasingly questioning whether he has focused on the quest for the White House to the extent needed for him to overtake front-running Vice President Bush.

"All the inherent strengths are there," said former labor secretary William E. Brock, who took over the chairmanship of the Dole campaign just a week ago. "But because of the demands on Bob's time and strength, the campaign has yet to achieve the focus it needs."

Central to the dilemma is Dole's determination -- reiterated in an interview yesterday with The Washington Post -- to hold on to his position as Senate Republican leader while challenging Bush and the four other GOP candidates in caucus and primary contests beginning just three months from now.

"Admittedly, it would be better to do one rather than both," Dole said in a telephone call from an auto rushing him from the Capitol to National Airport for an engagement in Peoria, Ill., last night. "But so far I've been able to do both, and as long as I stay in the leadership I get to help solve problems, and that's something the vice president can't say he's doing."

The dual role is physically demanding for the 64-year-old senator and, in the view of some supporters, is distracting him from the need they see to define his campaign message more clearly.

Richard B. Wirthlin, President Reagan's pollster and a recent recruit to the Dole campaign, said, "Not one candidate in 20 has the physical and mental capacity to do both jobs, but Dole is so well-endowed . . . I've advised him not to give it {the leadership post} up lightly."

Republican colleagues affirm there is no significant pressure on Dole to step down. Wirthlin, Brock and others cite the advantage Dole gains in publicity from his central role in foreign policy and domestic issues, which allows him to dramatize his contention that he is a battlefield commander, while Bush is only a titled dignitary. "The leadership role is a tremendous advantage, because there is such a void out there," said Edward J. Rollins, a former Reagan manager now heading the presidential campaign of Rep. Jack Kemp (R-N.Y.). "I would tell Dole absolutely not to give it up."

But Sen. Warren B. Rudman (R-N.H.), Dole's key ally in the first primary state, said, "It's clear he can't keep the active leadership role after January. It's not just the time demands. I've told him he's trying to represent too many constituencies: the Republican minority up here, the president and his programs. He needs to be representing himself."

In recent weeks, Dole has found himself juggling loyalties to the president, concerns of Republican senators and his own campaign imperatives on such ticklish issues as the Supreme Court nominations of Robert H. Bork and Douglas Ginsburg, the budget deficit package and the impending Intermediate Nuclear Forces (INF) agreement with the Soviet Union.

Sen. Charles E. Grassley (R-Iowa), the most prominent Dole backer in the first caucus state, said the dual role "has not been a problem to this point, but as Bush increases his time commitment to the state, I and others are wondering if Dole will be able to keep up."

A case in point came in the past four days. While Dole was tied up in closed-door negotiations on the deficit-reduction package, Bush was stumping through New Mexico, Texas, Michigan and Iowa. While Dole was making almost-every-night appearances on the network news, Bush was piling up the minutes on local television and on front pages from Santa Fe to Detroit.

Last night, Dole rushed off in hot pursuit of the Republican front-runner, scheduling stops in four midwestern states in barely more than 24 hours.

The catchup game is more than a matter of scheduling. Dole is also struggling to close the gap with Bush on organization and support. The most recent polls show Bush reclaiming the lead from Dole in Iowa and expanding his margin over Dole in New Hampshire.

While colleagues marvel at Dole's ability to maintain a near-perfect attendance record and the energy needed to manage the Republican side of the Senate, there are signs of strained resources. Brock said in an interview that he found the headquarters operation short of staff and short of space and is seeking a new location. Others in both Dole's Senate offices and at the campaign find themselves hard-pressed to keep up with the conflicting demands of what they term "Dole days."

Bill Lacy, the campaign's political director, said the turmoil at the top has not impeded field organizing. "In the past month," he said, "we had a series of regional strategy meetings with our operating people in the states, and it was very encouraging to see the kind of people who have signed up. It's a fresh, good team."

Less certainty is expressed by supporters and aides about Dole's mastery of the message phase of his campaign. The official view, expressed by campaign spokeswoman Mari Maseng, is that the Oct. 28 Houston "Firing Line" debate with Bush and the other four Republican contenders "did what we set out to do. It went a long way toward showing Bob Dole's more charming side."

But even Maseng conceded that in ensuing debates Dole will need to do more "to stake out our own territory and differentiate himself from the others, particularly Bush, both on the issues and on the question of leadership."

"He was too cautious," Grassley commented. "He proved to people he is not the slasher they remember from the debate in 1976 {when Dole, as Republican vice-presidential nominee, opposed Democrat Walter F. Mondale} but he's got to be more aggressive in the future."

Rudman agreed, saying, "His media advisers and close friends had chewed on him so much about not being too sharp, he overreacted and was too restrained. There's nothing wrong with being tough as long as you're not vicious."

There is dispute as to whether Dole's debate performance suffered because it was sandwiched into his demanding Senate schedule. "Someone said I looked tired," Dole commented yesterday. "I didn't feel tired, but maybe I didn't realize it."

The Houston trip illustrates the extreme tensions of Dole's two roles. All the morning and early afternoon before the first televised Republican debate, he was tied up in Senate business, flying to Houston just hours before airtime. He flew back to Washington after the debate, arriving at 3 a.m., was in the Senate from 8:30 a.m. until early afternoon, then flew to California for the biggest fund-raiser of his campaign and back across the continent again to be at an early-morning budget conference.

Brock noted Dole's mastery of the issues in the debate, but said, "He's learning that as a candidate you can't cover all the issues. You have to focus on three or four things of major media consequence. He's a legislator, but he's learning that a presidential candidate has to deal with a broader range of things and not all the details.

"It's a transition," Brock said.

Asked whether the transition would be speeded by Dole's shucking his Senate leadership role, Brock said, "There's no serious thought of changing it now."