The National League of Cities yesterday joined a chorus of criticism of a proposal for an "all-savers" tax-exempt certificate, which is moving swiftly through Congress and is backed strongly by the ailing savings and loan industry.

At the same time, House Speaker Thomas (Tip) P. O'Neill (D-Mass.) said that the fight over a tax bill came down to one point: "If you earn less than $50,000, you benefit by the Democratic bill. If you earn more than $50,000, you would want Reagan's bill."

O'Neill also indicated that the tax bill would not be completed by Congress and ready for President Reagan's signature by Aug. 1 as the president has urged. The House Ways and Means Committee will finish work on its tax bill by July 24, O'Neill added.

Meanwhile, Rep. Ken Holland (D-S.C.) one of the 29 conservative Democrats who supported Reagan on budget cuts, said yesterday that it does not appear that Reagan has the votes to pass his tax cuts in the House.

The president of the National League of Cities, William Hudnut, told reporters yesterday that the all-savers certificates -- which would offer a tax exemption on up to $2,000 of interest for a married couple -- would attract money from tax-free municipal bonds.

He chracterized the certificate, which has been approved in slightly different versions by both the House and Senate tax committees, as a "misguided and misunderstood bailout of the savings and loan industry that will cost cities and the average taxpayer billions of dollars."

The U.S. League of Savings Associations, a thrift industry group, countered by saying that Hudnut was "dead wrong." A spokesman said that the analysis underlying Hudnut's criticism was faulty it assumes that half of all municipal bonds were bought by households when official figures show a lower number.

The administration has opposed the all-savers certificate, although it also has said that the president would not veto an otherwise acceptable tax bill if the certificate plan were included.

There is considerable doubt about the certificate from staffers and some congressman on Capitol Hill. Sen. Robert Dole (R-Kan.), chairman of the Senate Finance Committee, has said that his committee is looking at other ways to raise savings and aid the S&Ls.

The next fight between Reagan and the Democratic majority in the House will be over the tax bill. The president wants three years of across-the-board cuts in individual tax rates, while the Democrats favor a two-year plan aimed at the lower-income and middle-income brackets.

O'Neill said yesterday that "we have a very good process going into effect in explaining what is in our bill" to persuade Southern Democrats who broke with the party over the budget to hold ranks. The Wall Street Journal yesterday reported that Ways and Means Chairman Dan Rostenkowski (D-Ill.) was expected to offer a sweetener to the southerners in the form of a tax exemption from the windfall profits tax for independent oil companies and royality owners.

Dole said that he believes there is room for compromise between the House and Senate on the individual and business components of the tax cut.

O'Neill denied charges that the Democrats are dragging their feet, pointing out that it was "the hugest tax bill in the history of the world," adding that "you can't get things done overnight."