Sponsors of one congressional tax-simplification plan defended it yesterday against charges that it would raise taxes, and authors of the other leading congressional version denied accusations that theirs is a revenue-loser.

There was also much talk of compromise at the House Ways and Means Committee's second day of hearings on overhauling the tax code.

Rep. Jack Kemp (R-N.Y.), sponsor of a modified-flat-rate proposal, said he he believes that congressional advocates of simplification and the Reagan administration will agree on a bill "representing a consensus upon the best features of the tax reform plans now on the table." Rep. Richard A. Gephardt (D-Mo.), who is pushing a simplification bill that would allow only three tax rates, said tax restructuring should be bipartisan, with all provisions on the negotiating table.

Adding to the positive tone, committee Chairman. Dan Rostenkowski (D-Ill.) said he is "very optimistic" about getting legislation through Congress. Sponsors of less-known tax-revision bills -- from a flat-rate tax to an across-the-board reduction in tax breaks -- said they hope Congress can summon the political will to reduce rates and broaden the tax base by removing deductions and credits.

But the hearing also served to underline some basic differences between the plan pushed by Gephardt and Sen. Bill Bradley (D-N.J.) and Kemp's proposal, which is cosponsored by Sen. Robert W. Kasten Jr. (R-Wis.).

Bradley said during the hearing that his bill, which has three rates of 14, 26 and 30 percent, would raise federal tax revenues by about $30 billion during its third year of implementation. The increase would occur mostly because neither tax brackets nor various deductions are indexed for inflation under the measure.

The Kemp-Kasten bill and the Treasury Department's simplification plan, which is being rewritten before being submitted to Congress, include inflation indexing for tax brackets and for the value of capital and depreciable assets.

Rep. Carroll A. Campbell Jr. (R-S.C.) said that, under the Bradley-Gephardt plan, inflation -- even if it remained low -- would push many taxpayers from the lowest bracket to the 25 percent rate during the first few years.

Gephardt conceded that the two had "identified what's going to be a raging debate in this bill."

But Kemp was questioned by legislators who weren't sure his plan would be revenue-neutral. The Kemp-Kasten bill proposes a flat personal tax rate of 25 percent, but with an exclusion of 20 percent of wages up to the Social Security maximum tax base, now $39,600.

Kemp said the overall bill would not change federal tax collections, but admitted that it probably would create a tax cut for taxpayers earning more than $100,000 "on a static basis. "We would expect that sheltering and loophole-using would be reduced," Kemp said. He added that the business portion of his tax plan would bring in the same amount of revenue as current law.

Although there are no solid numbers on the revenue effect of the most recent version of Kemp-Kasten, some tax experts have wondered how it can be revenue-neutral over a period of years. The bill includes depreciation provisions similar to those permitting firms to write off all investments in the first year, and it also allows full deduction of interest payments.

Together, those provisions would permit companies to write off more than the cost of the investment in cases where the investment is financed with borrowed money.

Rostenkowski hopes to move a bill through the committee this year. He said at the hearing that he believes public support for a tax overhaul is increasing as people become more aware that some companies and wealthy individuals pay little or no taxes.

"That's when the revolution comes," he said.