In an attempt to present a liberal alternative to President Reagan's economic plan, Sen. Edward M. Kennedy (D-Mass.) proposed a series of tax cuts yesterday aimed at aiding low-and middle-income taxpayers by raising personal exemptions and reducing the tax rate by 3 percentage points in every income bracket.

Saying that Reagan's tax plan provides "the most help to those who need it least," Kennedy also proposed eliminating the so-called "marriage tax" (which Reagan also has said he wants to eliminate, after his other economic measures are in effect), giving tax credits for home heating costs, lowering taxes on small businesses and increasing the depreciation deductions for high-technology industries.

In a speech prepared for the Massachusetts Municipal Association, Kennedy called Reagan's economic recovery program "a flawed plan" because "it fails the fairness test."

Low and middle-income people are beset by soaring inflation, energy bills, and Social Security costs, he said, "yet 30 percent of the Reagan tax cut goes to the 4 percent of Americans making over $50,000 a year, and less than 20 percent of the tax cut would go to the 60 percent of the taypayers who make less than $20,000.

"This year an individual with an income of $200,000 will receive a tax cut under the Reagan plan of $3,710," Kennedy added. "But a worker earning $20,000 a year will get a cut of only $114."

The speech, according to Kennedy aides, was an attempt to answer criticisms that liberal Democrats have offered no alternative to Reagan's economic package.

Specifically, Kennedy proposed:

Raising the current $1,000 personal tax exemption to $1,500 a year.

Reducing the tax rate 2 percentage points in every income bracket.

Adopting a depreciation schedule that would allow high-technology companies to write off 90 percent of investments the year they are made.

The writeoff for manufacturers would be 80 percent and that for new structures 60 percent.

Eliminating tax provisions that cause a married working couple to pay more income tax than if they had remained single.

Giving tax credits to individuals for home heating costs and businesses for conserving energy.

Giving special tax reductions to small businesses.

In his speech, Kennedy picked up a theme expressed frequently in recent days about Reagan's economic package: "There is a growing consensus in the Congress and the country about the broad goals of the economic policy."

"Democrats will do all we responsibly can to cooperate with President Reagan," he said. "We shall not be obstructionists. But neither shall we be rubber stamps."

Senate Minority Leader Robert Byrd (D-W.Va.) and House Speaker Thomas P. (Tip) O'Neill (D-Mass.) have said pretty much the same thing in recent days. "We all want the president to succeed," Byrd said yesterday at his weekly meeting with reporters in his Capitol office.

But he quickly added that Democrats don't want to be "stampeded" into any hasty action without ample time to study Reagan proposals. Byrd also charged that a television advertising blitz planned by Reagan supporters to build public support for the president's program "is a perversion of the political process."

"I think it is a sinister approach," the Senate minority leader said, asking later, "What is this going to be anyway, the buying of America?"

There may be disagreements among Democrats in the Senate about specific budget cuts, Byrd said. But he predicted they would be unified in opposition to the Kemp-Roth tax plan supported by Reagan, which would reduce income taxes by nearly one-third over a three-year period.

"We are not letting grass grow under our feet," said the West Virginia senator. "This party is not in disarray. It's not down in the mouth."

In a speech in Chicago Friday, O'Neill accused Reagan of trying to make "a squeeze play" on Congress.

"Reagan is pressing Congress for fast cuts in the most fundamental change in government policy in the last 50 years," while "giving only sketchy details" of his proposals and "hampering our efforts to retrieve more information," the House speaker said.

"We can't work using only righteous rhetoric," he added. "That's about all we're getting from them now."