AT LAST a slogan for the second term of Ronald Reagan. It's short, provocative. It's "Make my day."

It has everything, from Ronald Reagan's point of view. It was heard in a movie, spoken by Clint Eastwood and it's a shade more genteel than "Drop dead," which is what it really means.

"Make my day" is much used in the New York subway system, where life is raw and tempers are short and even brushing a fellow passenger's sleeve can lead to unpleasantness just short of an encounter with Bernhard Goetz.

That's the mood of the Reagan White House these days. Spring is coming on here, the sun shines brighter and longer every day, the forsythia is blooming. But in the corridors of power, there is anger -- and frowns and snarls.

Nobody is quite sure why.

At the beginning, onlookers suspected that the taunting and sneering was free-lance, occurring out of an excess of zeal by underlings. When David Stockman kicked the farmers while they were down, the immediate assumption was that the budget director was frazzled from too many hours crouched over his computer printouts.

But this week, there were signs that the baiting starts at the top.

After the Senate Budget Committee rejected his budget figures, the president went after the senators as if they were Sandinistas. They had presumed to cut defense spending and maybe raise taxes.

"I have only one thing to say to the tax increasers," said the president in the best Clint Eastwood manner. "Go ahead and make my day."

The new thumb-in-the-eye approach was originally credited to the new chief of staff, Donald Regan, who as Treasury secretary perfected a confrontational style that was the antithesis of the suaver method of his White House predecessor, James Baker. Others traced it to the advent of Patrick Buchanan, Nixon's old speechwriter, Reagan's recently seated director of communications and a true believer in the disembowelling, whodyathinkyouare school of dealing with opposition.

On Capitol Hill, whenever there was an administration witness, there was instep- crunching, gouging and, often, the shouted exchange.

What was going on? Did Regan and Buchanan haul in the Cabinet members every morning, make them take their macho pills, remind them that Reagan won 49 states and send them up to Capitol Hill with orders to take no prisoners? Was there a point system? Ten points, say for questioning a dissident's patriotism and four for getting a congressperson to yell at you?

Whatever the reason, the Reagan men roamed the city with fire in their eyes. George Shultz roasted scientists who dare to disagree about Star Wars, Caspar Weinberger did cutthroat work on defense budget differers and Clarence Pendleton, chairman of the Civil Rights Commmission, who is black himself, savaged black leaders who have criticized Reagan's civil rights policies. He called them "racists."

Strong stuff, that, and relics of another era when a president winced at seeing a constituency being dumped on, and spoke earnestly of their worry that the president would be "hurt" by so belligerent a champion.

But that was before we learned that Ronald Reagan loved it. He called Pendleton to praise him.

It was another signal that the chief himself is setting the tone.

During the first term, zealots like James Watt, who pioneered constituency-bashing by regularly excoriating the environmentalists whose interest he was supposed to protect, pleaded, "Let Reagan be Reagan."

Now Reagan has decided to be himself. With no more elections facing him, he can let his contempt for the Democrats and his irritation that anyone should tamper with his mandate all hang out.

Observe that he gives the bashers not only the praise, but the pork, too. In a power struggle between political director Edward F. Rollins and Buchanan over the custody of the public liaison section of the White House, Buchanan won. With his new turf, he gets 38 new job slots. You can be sure no wimps need apply. Rollins, who ran Reagan's reelection campaign without giving offense to anybody save the citizens of Minnesota, was always considered a model of conservative partisanship. Apparently, in the new militancy, he is too moderate. Rollins has been working quietly and industriously at "realignment," an ambitious project aimed at transforming the Republicans into the majority party. He is an advocate of courting constituencies rather than cuffing them into line. That's heresy now.

Reagan has always been, under the nice- guy exterior, a combative politician. And, as he said, after he won, "You ain't seen nothin' yet." It's flak jackets and steel helmets until everyone gets with the program.