WHEN President Bush hits the road, he travels in a sort of cocoon within a cocoon.When halfway around the world, Mr. Bush is surrounded largely by the same day-to-day advisers, and most of the same reporters, as at home in the White House. As the cocoon moves, the heart of Washington seems to move with it. Last week, the cocoon flew to Moscow and Kiev, but it took its business with it. The drive for a peace conference in the Middle East was the most pressing business of the week. Inevitably, it gravitated to the center of the Moscow summit agenda as well. When Bush dropped in on the handful of reporters riding in the back of Air Force One as it taxied to takeoff for Moscow, the first question he was asked was whether he had any word from Israel. His last word with reporters at the end of the trip, just before the plane took off from Kiev for Washington, was to welcome the news that Israel had accepted the invitation for a peace conference. In Moscow, the president's cocoon was tight and thick. The traveling White House press, rubbing elbows with the same colleagues and White House press aides as in Washington, could easily have worked the trip without laying eyes on either Bush or President Gorbachev in person. In most cases, the only reporters who actually see the comings and goings of the summiting leaders are in small rotating "pools" that write reports and pass them out to their colleagues. And with every practical detail arranged by White House advance teams, it would have been easy for the working press never to slip out of its self-contained cocoon into the surrounding city. Even the press telephones in Moscow were set up as direct American lines. This reporter had the same area code and local exchange in Moscow as at home in suburban Virginia. For Bush's part, he made some brief attempts at stepping out to greet passing Muscovites, but the summit seemed to arouse little interest from Moscow civilians. He was not entirely insulated from all the maneuverings of Soviet politics, however. Russian Federation President Boris Yeltsin, of course, stood up Mr. Gorbachev's luncheon invitation with Bush. More awkward was the dinner Mr. Yeltsin did attend. The dramatic rival to Gorbachev arrived last and commandeered the wives of both presidents for the gathered cameras, leaving Bush and Gorbachev to occupy themselves chatting and pondering the chandelier overhead. The cocoon was thinnest during the few hours Bush spent in Kiev on Thursday. In contrast to more jaded Muscovites, the Ukrainians greeted the presence of the leader of the Western world with obvious enthusiasm. His very presence was taken as an acknowledgment of Ukrainian aspirations to independence from the Soviet Union. People gathered along the motorcade route for miles, smiling and waving. Feelings ran strong. For years, it was considered provincial to speak Ukrainian instead of Russian. Now, reporters found that many Ukrainians refused to speak to them in Russian out of nationalist pride. That afternoon, laying a wreath at Babi Yar, a memorial of an atrocious episode of the Nazi holocaust, Bush appeared choked with emotion toward the end of his unusually poetic speech. That sense of time and place disappeared quickly enough. Mr. Bush headed directly from Babi Yar to Air Force One. On the flight back to Washington, he signed a letter and handed it to reporters in the back of the plane: a veto threat addressed to the Speaker of the House on a proposed gas tax.