Forewarned to avoid slouching and chewing gum on camera, the U.S. Senate took a cautious first step into the television era yesterday with the start of a one-month trial of live, closed-circuit coverage of its proceedings.

The first reviews were decidedly mixed.

Senate Majority Leader Robert J. Dole (R-Kan.) called it the start of a "historic journey" and suggested that the inaugural debate -- on the fiscal 1987 budget -- might be so exciting it should be "X-rated."

But House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) was less impressed. After watching the first few minutes, devoted to speeches on Law Day, Passover, the economic summit, the nuclear arms race and former budget director David A. Stockman, he said, "It's unbelievable. You wouldn't believe there could be so much wind from so few people."

The wind had reached gale force within an hour or so, however, and it was aimed straight at O'Neill -- a theatrical warm-up, some suggested, for what may happen when the televised Senate goes public next month. With members of the House as his only outside audience, Dole took off after House Democrats for an earlier budget maneuver with a display of outraged innocence, righteous indignation and bold counterthrust that appeared aimed at boosting the Senate's ratings, politically if nothing else.

After months of fretting over whether television would undermine its institutional traditions, the Senate voted earlier this year to experiment with broadcasting its proceedings, starting with radio coverage in mid-March and in-house video tests over the past week.

The gavel-to-gavel closed-circuit coverage for the Capitol complex will last through May. On June 2, day-long coverage of the Senate will be made available to the public, carried in full by the cable television C-Span network and available for use by the larger networks. In mid-July, the Senate is expected to decide whether to continue television on a permanent basis.

The current operations are being conducted with leased equipment, including robot cameras of Japanese, British and French heritage that are operated from a control room in the basement of the Senate.

Although the cameras swiveled around gracefully to focus on their targets, some of the stars of the drama had troubles with their scripts. Dole, for instance, forgot to turn on his microphone for several minutes at the start of the session.

But the Senate did find a way around one of its most difficult television problems -- what to do during an endless, boring quorum call, which is one of the Senate's most official-sounding, but least interesting, procedures. Because a quorum call is essentially a delaying tactic, during which the clerk calls the roll at foot-dragging pace, there is nothing to look at in the chamber. The solution: 46 minutes after the session opened, when the Senate fell into a quorum call, all a viewer saw was the message, "The Senate Is Conducting a Quorum Call" sprawled across a blank screen.

It was Dole who issued the fatherly advice to his colleagues to hold their heads up high (so the cameras could focus on faces rather than on balding domes) and "not to chew gum and things like that." Most of his flock took heed, looking marginally spiffier and more attentive than usual. Dole had already arranged for lecterns for himself and Minority Leader Robert C. Byrd (D-W.Va.) to help them keep a level eye on their audience rather than down at their desks.

It was also Dole who gave the final verdict on the Senate's television debut. "I'm not sure about the debate," he said, "but I'm advised we look good."