The tone is calm and reasoned, but the message attacking Ronald Reagan is harsher than anything ever said under the imprimatur of George Bush and his presidential campaign.

"Can we afford the same mistakes twice?" asks the mellifluous radio voice in Bush's new commercial. Trying to recapture the initiative in his topsy-turvy battle with front-runner Reagan, Bush's latest commercial charges that Reagan, like Jimmy Carter before him, cannot be entrusted with the dangerous problems of U.S. foreign policy. "He has no real understanding of the dangers we face in the decade of the '80s. . . . He didn't even know who the president of France was," says the nameless voice on the radio.

Written to catch the tail end of a mild presidential primary campaign here, the twinning of Reagan with Carter was blessed with fortuitous timing. Carter's slapstick comedy in the affair of the U.N. Security Council vote was in the headlines just as the Bush ad was being beamed to Florida voters for the first time.

That Bush has to take off the gloves of gentlemanly conduct is beyond dispute if he is to have a chance to hold down Reagan, much less overtake him, in the important southern battleground of South Carolina on March 8 and Florida, along with Alabama and Georgia, on March 11.

Here in Florida, Reagan appears headed for a modest win in a major-state primary that got away from him in his 1976 contest with President Ford.

Now, most of the signs point the other way. Reagan should do even better in his 1976 stronghold of north Florida and the ever-growing Cuban-American community in Dade County (Miami), which turns out nearly 30 percent of the county's Republican vote. And Bush, lacking the political clout of the White House, is weaker across the central Florida Republican heartland, from ever-growing Republican populations in the west coast's Tampa Bay area to Palm Beach on the Gold Coast.

When Reagan spent one day last week mining Republican votes in Sarasota, Sun City and the suburbs of Tampa Bay, he drew large and enthusiastic crowds, surprising some politicians. To avoid a damaging defeat, Bush must do well in the central Florida regions captured by Ford.

Bush's hot new radio commercial, his managers hope, will turn up the juice on the quiet debate that has dominated the Florida race so far ("It's boiling down to a publicity contest without issues," a Republican committeeman in Flagler County told us after hearing Bush address a Daytona Beach Lincoln Day luncheon Feb. 5).

Underneath, however, it is not so quiet. Reagan surrogates, with or without his consent, have choked the mails with fat droppings, castigating Bush for the sin of once belonging to the Trilateral Commission. The commission is a group founded by David Rockefeller, chairman of the Chase Manhattan Bank, that is devoted to improving trade and economic welfare of the United States, Japan and Western Europe.

Although the attacks are carbon copies of the job so neatly done on Bush in New Hampshire, he told reporters in Daytona Beach that they were "not giving me any major political problems." Perhaps. But one Bush backer, state Sen. Van B. Poole, bitterly complained in a private letter last month to Reagan campaign chairman Tommy Thomas that Thomas was far out of line when he publicly called Bush a candidate "controlled by David Rockefeller and other one-worlders." Thomas sent no reply.

Ironically, Reagan has been helped here by Ford's abrupt attack on him as one Republican who just could not get elected president. Even committed Bush supporters were shocked. Bush himself, clearly aware of this unexpected time bomb for Reagan, sought to disarm it by professing confidence that Reagan not only could get elected but could actually serve a full eight years in office until the age of 77.

Ford is fast becoming the third man in the Republican race, eclipsing John B. Connally, Rep. Philip Crane and even Rep. John Anderson. Connally and Crane are no more than potential vote-stealers from Reagan, and Anderson will not find liberals or independents here out of whom to make a Massachusetts-style bundle.

Indeed, if Bush's new attack on Reagan fails to raise the level of combat, the Florida primary, so crucial in some past presidential campaigns, may not signify much of anything. It may signify far less than the prospect that Ford may make a spring tryout soon after the single most important classic thus far -- the Illinois primary March 18.