The country has "not mortgaged the future" to pay for its 70-month economic expansion, the chairman of the president's Council of Economic Advisers said, countering Democratic political charges.

Beryl Sprinkel, the chairman of the advisory group, predicted a cooling economy would stabilize inflation and interest rates for the rest of this year, if present policies are continued.In a briefing for reporters at the White House Wednesday, Sprinkel denied any political motive for his rosy report on the economy and refused to comment on the economic proposals of the two candidates for president.

He went on, however, to tout the Reagan expansion, which is now in its 70th month and already one year longer than any other peacetime expansion in U.S. history, as "real, substantial and sustained."

"We have not mortgaged the future . . . so long as the basic thrust of present policy remains intact," Sprinkel said. "Expansions do not die of old age; they die of inept policies."

Previewing a speech he was to give later in Pittsburgh, which he called his second annual update on the state of the economy, Sprinkel also hailed administration moves to keep inflation under control. "We're not making the mistake of putting the foot on the accelerator at this point," he said.

He also conceded that "more (effort) is needed" to balance the burgeoning federal deficit.

"Considerable progress has been made, and if we stick to the Gramm-Rudman targets, additional progress will be made until we get the deficit down to zero," he said.

Reagan's "approach has worked," Sprinkel said. "I'm not arguing that nothing else would work, but this clearly has worked."

On Tuesday, while officially maintaining it was "not appropriate" to discuss politics, Sprinkel endorsed Vice President George Bush's presidential campaign promise not to raise taxes.

"A tax increase would probably raise revenues," Sprinkel said. "But we would end up with a larger government and a weaker private sector with slower job growth and more unemployment."

Sprinkel denied claims made by Democrats in this presidential election year that the middle-class has been squeezed out of the benefits of the improvement, while the poor have languished in poverty.

"It's a matter of interpretation," Sprinkel said, while acknowledging that he had "no real good explanation" for the stubbornly stable poverty rate.

Sprinkel cited statistics showing hourly compensation, including fringe benefits, for full-time, full-year employees have risen 12 percent since 1982, after adjustment for inflation.

He said studies by the council's staff show that, contrary to Democratic claims, 92 percent of the new jobs created in the entire expansion are full-time jobs and two-thirds of the new jobs have been in the higher paying occupations.

Addressing the Republican "gender gap" problem, Sprinkel said his studies showed women in particular have profitted during this economic expansion, increasing their weekly earnings by 27 percent compared to a 19 percent rise in earnings for men.