The principal Democratic and Republican sponsors of tax simplification promised to work together for the cause yesterday as they re-introduced two separate simplification bills.

The air of the crowded hearing room was thick with bipartisanship as Reps. Richard Gephardt (D-Mo.) and Sen. Bill Bradley (D-N.J), sponsors of one plan, and Rep. Jack Kemp (R-N.Y.) and Sen. Robert W. Kasten Jr. (R-Wis.), who have another, said both of their bills would make the tax system simpler, fairer and more efficient.

"Obviously, we each believe our own proposals offer the most effective way to accomplish tax reform," said a statement by all four legislators. "There are differences, but more important are the similarities, which provide a common ground to forge a tax system that the American people deserve."

The sponsors emphasized their willingness to compromise, with Kasten declaring that "there's nothing in this bill that's not negotiable." And Gephardt said the important thing was to find a simplification package that "can get 218 votes in the House and 51 votes in the Senate and the president to sign the bill. That's all that counts."

Nor, the legislators said, does the fact that tax simplification will be put on hold until Congress deals with spending cuts hurt its chances. Senate Republicans are determined to push for spending cuts before tackling the complex issues involved in restructuring the tax code.

But reducing tax rates, expanding the tax base by eliminating deductions and credits and enlarging personal exemptions, the sponsors said, will take presidential leadership in the face of strong opposition from many interest groups. And they emphasized their strategy of seeking broad, grass-roots support in order to nullify the many Washington lobbying groups opposed to one or another provision of the three simplification measures.

"This is an issue that's not going to be decided in the Beltway," said Kasten.

So far, President Reagan has endorsed tax simplification only in principle. He is expected to do so again in his State of the Union address next week, but his administration plans to negotiate with congressional tax-simplifiers before officially proposing legislation to Congress.

Separately, Sens. Dennis DeConcini (D-Ariz.) and Steve Symms (R-Idaho) introduced a bill calling for a 19 percent flat-rate tax so simple that the form takes up one side of a postcard. It also included a larger personal exemption -- $8,000 for a single head of household -- and the sponsors said the measure would reward savings, investment and productivity.

The Bradley-Gephardt and Kemp-Kasten plans, however, are generally considered the two most viable simplification plans, along with the one proposed last November by the Treasury Department. The three differ less on provisions that affect individual taxpayers than they do on specifics of particular interest to various business groups.

Bradley-Gephardt has three graduated rates of 14 percent, 26 percent and 30 percent, while the Kemp-Kasten bill, which was revised before being introduced in this Congress, sets a flat rate of 24 percent. That would be somewhat graduated, however, because 20 percent of salary up to the amount of the Social Security wage base ($39,300 this year) would be excluded from tax.

The Treasury plan, which officials have said is also subject to negotiation, sets three personal tax rates of 15 percent, 25 percent and 35 percent.

The three preserve some of the most politically popular deductions, including the one for mortgage interest, although under Bradley-Gephardt the deduction could only be taken at the lowest rate, 14 percent.