President Reagan moved on several fronts yesterday to fire up his campaign for the $98.9 billion tax increase, declaring that "those who are opposed to it cannot hide from the fact that they are supporting increased deficits and higher interest rates."

He devoted most of his working day to events designed to pressure House Republicans to support the bill, now in a House-Senate conference committee, and to advance the election-year theme that the legislation is really tax reform instead of new taxes. White House officials said Reagan was close to a decision to make a nationally televised speech on behalf of the tax bill later this week, but no final decision had been made.

"It is being dishonestly tagged as the largest tax increase in history," the president said as he began the last of four separate lobbying sessions with congressional Republicans. "It may be the greatest tax reform."

At the same time, Treasury Secretary Donald T. Regan warned that failure to approve the spending cuts and tax boosts could lead to a GOP donneybrook in the fall election campaigns. "Suppose we do nothing," he said. "You'd have Republicans having to run on a platform of deficits in the neighborhood of $165 billion a year over several years."

The House-Senate panel reached tentative agreement yesterday on spending cuts in the Medicare and Medicaid programs but had not resolved such major issues as corporate tax leasing or business tax increases.

The president summoned a few industrial leaders and 65 congressional Republicans to the White House to reinforce his message that the tax bill was not a reversal of last year's supply-side tax-cutting efforts, but instead represents tax reform that will reduce interest rates "and get the economy moving again."

"Only 17 percent of this bill represents actual tax increase, and 31 percent represents simply better compliance and collecting money from people who under existing law . . . are not paying," the president told GOP lawmakers. "And the rest happens to be actual reform correcting unintended benefits or advantages that have come into being through legislation here and there . . . "

Reagan talked with both adversaries and allies yesterday in his push for the tax bill. He shook hands with Rep. Jack Kemp (R-N.Y.), who led the conservative tax rebellion, in what was described as a friendly but brief White House encounter after a meeting with the House Republican leadership. "Jack sincerely believes -- is a purist in supply-side economics," Reagan said later. "We'll continue to talk and reason together." The president's comments contrasted with the sharp criticism of Kemp in recent days from some White House officials who blamed him for leading a revolt against the tax bill.

Asked whether he personally thought Kemp was running for president, Reagan responded: "I didn't know the job was up for grabs."

After meeting with Reagan alone, Sen. William V. Roth Jr. (R-Del.), coauthor with Kemp of the tax cut Reagan won from Congress last year, declared his support for the tax bill as "the best reform package to come before Congress" in many years. Roth said in a letter to Reagan that "cries of outrage by my fellow 'supply-siders' " over the tax bill "simply don't hold water."

But the president still ran into resistance from a "core" group of House Republicans who were among his earliest supporters. Rep. Thomas B. Evans Jr. (R-Del.), a leader of the group, said they had asked for modifications in the tax bill as the price for their support. "All of us are looking at what can be modified," he said.

Attempting to overcome business opposition to the tax bill, the White House won endorsements from a coalition of industry groups led by the American Business Conference.

Chrysler Chairman Lee Iacocca also put in a pitch for the legislation after seeing the president. "It's time to move, we need action, and as I said, if you can find a better tax bill, grab it."

U.S. Chamber of Commerce Chairman Paul Thayer said the organization's 64-member board would back the tax bill next week. The chamber's executive committee recently had come out against it.