Sen. Edward M. Kennedy (D-Mass.) assailed Ronald Reagan and the Republican Party yesterday for drafting a platform whose "planks will come crashing down on hard-working middle-income Americans."

In a speech here to a policemen's union, Kennedy continued his quixotic quest for the presidency, with little mention of his opponent, President Carter.

The organization, the International Union of Police Associations, is to make its presidential endorsement public this Thursday and is expected to join the Kennedy camp.

"We've been the down guys all our lives, and even if it's a futile gesture, we don't walk away from our friends," said IUPA President Edward J. Kierman, who serves on the Kennedy campaign's labor committee.

Kennedy's longstanding and outspoken support of police officers' bread-and-butter issues matters much more to the 30,000 IUPA members than Reagan's law-and-order rhetoric.

"Policeman nowadays are more interested in how much money they make than in whether some criminal gets the death penalty," said Len Doran, president of the Burbank local of the IUPA.

The group's success in attracting members lends credence to the views expressed by presidents of IUPA locals representing 42 states.

Less than two years ago, the IUPA became the newest AFL-CIO affiliate.

Since then, it has organized 30,000 sheriffs, police and other law enforcement officers in 158 locals. It has chapters in the District of Columbia, Houston, San Francisco, Chicago and Cleveland, among others.

Thirty IUPA organizers now are trying to add more of this country's 450,000 police officers to the union rolls.

Reagan has little appeal for the IUPA membership. They do not seem to buy the Republican Party's sales pitch to blue-collar workers.

"Reagan is the candidate of management, not of labor," said one.

Some worry that Reagan's election would signal a return to the turmoil of the 1960s, when policemen "became the scapegoats because they had to defend the ultraconservative, unpopular policies of a president."

The choice for these policemen boils down to Carter and Kennedy. Kiernan says that Carter has not been in touch with his union, but notes that Mondale may make an appearance at the union's convention.

In his speech, Kennedy pointed out that he has voted five times for group life insurance for police officers. In each case, he said, the bill had failed "because of opposition from the administrations of Richard Nixon, Gerald Ford, and Jimmy Carter."