House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and other Democrats lashed out yesterday at a new Republican television ad campaign on Social Security, with O'Neill calling it a "multimillion-dollar" effort to "lie to the American people."

The campaign is being financed by the Republican National Committee in 58 markets. It contains TV spots declaring that President Reagan "kept his promise to the American people" to guarantee that a 7.4 percent automatic cost-of-living increase in monthly Social Security benefits, due to start last week, would be paid on time and in full.

O'Neill, in a statement and backup papers issued by his office, said that in fact, on May 12, 1981, President Reagan had proposed a three-month delay in the increase and this year, during budget negotiations, administration officials had again proposed delays.

The backup papers also indicated that administration officials had raised the possibility of "capping" the increase at 4 percent instead of 7.4 percent.

O'Neill said the only reason that had not happened was "because we in Congress refused to go along with it."

"I don't know what everybody's complaining about," said GOP national chairman Richard Richards. "We've just created a nice little commercial."