The only conceivable obstacle now to a new, get-tough George Bush willing to underwrite direct attacks on Ronald Reagan's age is George Bush himself, raising the Prospect that Marquis of Queensbury rules will yield to brass-knuckle street fighting.

"We were warned that George would go on the attack here," a Reagan ally told us after the minicing Republican debate last Thursday, "but we didn't think he would. That's not George's style."

But the shock of Bush's lopsided loss to Reagan in New Hampshire has rendered logical conclusions obsolete. A series of new television ads, insulating Bush himself from onerous griatric judgements about Reagan's 69 years, was ordered up from Bush's media czar, Robert Goodman, before the Thursday night debate. Following the man-in-the-street interview style, thesecommercials will dramatize the age issue by letting senior citizens themselves discuss it -- to Reagan's disadavantage.

To some Bush intimates, this assuredly is not the expected to style of George Bush, whose self-control and stubborn political decency have angered some of his handlers. But when that fleeting euphoria between Iowa and New Hamphisre evaporated, drastic remedies were called for.

The consensus was finally reached, after several abortive sessions, at a three-hour, closed-door strategy talk in the Ramada Inn here Tuesday afternoon, before the debate that night. Campaign chairman James Baker and national political director David Keene, strongly backed by Goodman, insisted on the change in strategy. Bush listened, ordered the TV spots to be made but withheld final approval for running them.

For weeks Bush has systematically exploited Reagan's issue, always unwilling to bring it to a boil. Arriving here for the debate last week, for example, Bush headed for a University of South Carolina field house. There he changed into running suit and sneakers and led a pack of students on a three-mile jog.

Trying to drive the point home, he told a student rally at the Capstone Dorm that Jimmy Carter's jogging was essential to the health of any president. As for himself, Bush said he was physically prepared to lead the nation from the White House for "eight full years." No mention of Reagan.

"I feel about 35 years old and ready to charge," Bush told a fund-raising dinner in Boston's Park Plaza Hotel a Wednesday night before flying here. Such indirect jabs at the fact that Reagan would be the oldest man elected president in U.S. history falls short of what Bush's handlers want. They are convinced that concern about Reagan's age is highest among his own generation -- the same senior citizens who will soon be speaking out on Bush's TV commercials, if Bush flashes the green light.

Hitting the front-runner on age could backfire on George Bush, both creating anger in Reagan's granite-hard base and depicting Bush himself as a backstreet fighter. But Bush came out of his debacle in New Hampshire livid over what he termed "non-stop attacks" on him not by Reagan but by Reagan's surrogates. The most effective were William Loeb's Manchester Union-Leader and Reagan's brainy state chairman, Jerry Carmen. "They told me no one has ever taken such a non-stop pounding from Bill Loeb," Bush confided to a friend shortly after the New Hampshire vote was counted. He is convinced that Reagan approved the assault, particularly Carmen's brilliant explitation of the class issue, which portrayed Bush as being backed by the silk-stocking country-club set and Reagan by blue-collar workers.

One Bush aide speculates that a new gut-fighter image for Bush might spawn its own reward, a hope that gathered force after another dose of Bush's gentlemanly conduct in the debate here. "Now I know who Bush reminds me of," one Republican told us after that debate. "He acts like Adlai Stevenson."

A softer verdict, but in the same vein, came from a member of Bush's own campaign staff. "Too subdued," he said of Bush's performance, referring particularly to his failure to exploit Reagan's lack of experience in foreign and national security issues.

If Bush now steels himself to take the offensive, particularly on the age issue, that verdict will reversed -- and Bush may turn another corner in the fascinating Republican presidential race.