As George Bush approaches the high noon of his quest for the Republican presidential nomination, he is striving to move out of the shadow cast by President Reagan while seeking to capitalize on Reagan's enormous popularity. Reagan's shadow has already shortened for Bush. These days he is seldom dogged with questions about distancing himself from the president. Instead, the questions now center on whether his candidacy is exerting too much influence over the Reagan White House. Both Bush and Reagan in recent days have fended off suggestions that some presidential decisions have been motivated by an effort to help the Bush campaign. And confronted with public opinion polls showing Bush far behind among Democrats who voted for Reagan, Bush advisers have now switched strategy to make sure that the Reagan shadow doesn't disappear altogether. After first planning no contact at the Republican National Convention between Reagan and his vice president, a joint rally is now scheduled for Tuesday in New Orleans. It follows an upbeat joint appearance between Reagan and Bush at a rally last Friday for senior administration officials at the Old Executive Office Building, during which Reagan lavished praise on his vice president. If all goes as planned, Bush's transformation into his own man will be complete when he accepts the Republican presidential nomination Thursday night at the Republican National Convention, according to aides and GOP consultants. By the end of the convention, ``all the emphasis is going to be on George Bush and his vice presidential nominee,'' said his chief of staff, Craig Fuller. Lee Atwater, Bush's campaign manager, said that Bush ``has made a conscious decision for seven years to be secondary to the president.'' ``And now that he's running for the president himself, he's talking about how he feels, what he believes in, and where he wants to take the country,'' Atwater said. To be sure, there are some areas in which Bush does not want to put much light between himself and Reagan _ largely the current economic expansion, now in a peacetime record of 69 months, and the fact that most of the world is at peace. The vice president has been inching away from his close identification with administration policies since he and aides huddled for a week in late May at his vacation home in Kennebunkport, Maine. The consensus from his advisers was near-unanimous: he had to do more to establish his own agenda. Since then, Bush has hinted that he would be more aggressive in dealing with the Soviets, would enforce civil-rights laws more vigorously, would spend more on education and do more to clean up the environment than the president he has served for the past 7{ years. He has also proposed many of his own initiatives, including a $2.2 billion child care program, a ban on offshore ocean dumping, and a reorganizaton of the anti-drug effort under the office of vice president. He also has proposed a new White House ethics office and a ``flexible freeze'' on federal spending. But most of Bush's moves away from the administration have been cautious, for fear of appearing disloyal. ``I'm not going to change just to emerge from a shadow,'' he has said. In a recent interview, Bush said he will differ with Reagan when he feels he has a different set of goals. ``I have every intention of saying, here is what we've done and and here are my priorities. ``But I'm not doing that to quote distance myself, to show my own identity,'' he continued. ``I'm doing it to say, look, here is what we've done. Here are the successes, and here are the shortcomings, and here's what George Bush will do.'' Still, Bush's close identification with the administration continues to pose problems for him as Democrats keep driving home their ``Where is George?'' slogan. When the general election campaign heats up after Labor Day, Bush is expected to be hit repeatedly by Democrats for his role _ still vague _ in some of the less popular Reagan administration decisions, including arms-for-hostages dealings with Iran and past support for Panamanian dictator Manuel Noriega. Meanwhile, the impact of Bush's candidacy has already been seen in a number of recent presidential decisions _ including Reagan's selection last week of a Hispanic, Lauro F. Cavazos, to be education secretary _ fulfilling in advance a Bush campaign pledge. Reagan filled two other Cabinet vacancies with people on close terms with Bush, naming former Pennsylvania Gov. Richard Thornburgh to succeed Edwin Meese as attorney general and Wall Street investment banker Nicholas F. Brady to follow James A. Baker III as treasury secretary. Baker resigned to run Bush's campaign. The president also yielded to Bush's urgings in deciding to allow a plant-closing notification bill to become law and in the president's veto of a $299.5 billion defense authorization bill. Reagan also has picked up on some of Bush's campaign themes, including advocating the death penalty for ``drug kingpins.'' ``I don't think it's as much a matter of Bush disagreeing or separating himself from Reagan as much as demonstrating that his agenda is a little different,'' said GOP consultant Charlie Black, who is a Bush convention adviser. And John Sears, another Republican consultant, said: ``The real moment of truth is when Bush accepts the nomination.'' ``He can campaign on the virtues of what eight years of the Reagan administration have brought to the country. And he can prove himself presidential. In his acceptance speech, he can do it all,'' Sears said.