House Republicans told President Reagan yesterday that they have marshaled enough votes to sustain a veto if Congress attempts to "tamper" in any way with the remainder of his tax cut program, including the 10 percent income tax cut scheduled for July and future tax rate adjustments for inflation.

Their action came as a Senate Republican task force, groping for a consensus on the budget, reportedly reached tentative agreement on defense and domestic spending but continued at odds over taxes, the Senate Republicans' biggest problem all along.

Sources said the senators were willing to go along with an after-inflation increase in defense spending of up to 7 1/2 percent for next year, halfway between Reagan's request for 10 percent and the Senate Budget Committee's recommendation of 5 percent.

They also indicated support for a trim of $3 billion to $4 billion from the committee's proposal for domestic spending, which is still more than Reagan wants. But they remain split on taxes, including whether to lock in a big increase for fiscal 1986, sources said.

In their letter to Reagan, 146 House Republicans--just enough to sustain a veto--called on the president to pledge "unequivocally" to veto any such legislation and vowed their support in sustaining him.

Over strong opposition from Reagan, both the Democratic-controlled House and Republican-led Senate Budget Committee have called for tax increases sufficient to accommodate repeal of both the July tax cut and indexing, including a $30 billion tax increase for next year.

In addition, Democratic leaders of both the House and Senate, joined by some Senate Republican moderates, have advocated repeal or modification of the two provisions, although Senate Republican leaders predicted on Wednesday that both tax laws would remain intact.

"We view with deep dismay the increasing pressures to repeal or delay the 10 percent personal tax rate cut scheduled for this summer and the tax rate indexation provision scheduled to take effect in 1985," said the 146 Republicans, who included Minority Leader Robert H. Michel (R-Ill.), Minority Whip Trent Lott (R-Miss.) and other party leaders.

The letter's signers also included Republican House members from the Washington area.

The House GOP effort to shore up support for Reagan's tax program came as Sen. Thomas F. Eagleton (D-Mo.) introduced what he called a "compromise" on indexing that would postpone the automatic inflation adjustments until the federal budget deficit is reduced to 2 percent of the Gross National Product, the key measure of the country's economic output.

The deficit would not drop to this level until sometime after 1988 under administration projections, Eagleton said.

This "trigger" is similar to one proposed by Reagan for standby tax increases totaling about $150 billion for fiscal 1986, 1987 and 1988. They would take effect only if deficits continued to exceed 2.5 percent of GNP and if the economy is growing.

Eagleton estimated his proposal would save $90 billion through 1988.

"Quite frankly, this bill represents a compromise of my own feeling that indexing was a terrible mistake and should be repealed outright," said Eagleton. He characterized the inflation adjustments as a "lifetime mortgage on the ability of the government to function," but said compromise is necessary to control deficits.

Leaders of the House Republican effort argued, however, that Reagan's tax cuts, including indexing, are not responsible for big deficits. "What Congress needs to do...is to concentrate on getting control of the spending side of the budget," said Rep. Connie Mack (R-Fla.), who spearheaded the letter campaign.

But Mack said the other signers had not committed themselves to cuts in spending. Nor, he said, did they necessarily rule out other tax increases to help reduce deficits.

Meanwhile, Senate Democrats came out against Reagan's cost-cutting proposals for Medicare, charging that they would shift the burden of soaring health costs to the elderly. They criticized his proposals for higher out-of-pocket fees to cover doctors' charges and for hospital fee changes to cover the cost of catastrophic care.