U.S. Attorney General Dick Thornburgh said Monday he has no reason to believe he will be selected as Republican George Bush's running mate, and that it would be ``presumptuous'' to say whether he would accept such an offer. ``I'll be back on the job tomorrow in Washington,'' Thornburgh, the former two-term, Pennsylvania governor, told reporters at a rally sponsored by his state's GOP convention delegation. Thornburgh said he was in town only to hear President Reagan's address to the convention Monday night. Asked how he would respond to an offer from the all-but-certain nominee for the vice presidency, Thornburgh hesitated before replying: ``That's a presumptuous kind of a statement to make. I have no reason to think that's going to happen.'' Thornburgh was sworn in as the successor to former attorney general Edwin Meese III on Friday. Sen. Arlen Specter, R-Pa., a member of the Judiciary Committee that heaped praise on Thornburgh during his confirmation hearing, said senators would understand if Thornburgh left after less than a week on the job. ``Obviously the Justice Department is very important. We pushed hard to move things quickly (with confirmation),'' Specter said. ``But my sense is people would feel this is important _ if the vice presidency gives him a higher calling, that's that,'' he said. Specter, who favors Kansas Sen. Bob Dole for the No. 2 spot on the ticket, said he did not know what Thornburgh's chances were of getting the nod.