Congressional Democrats offered the country a mixed reaction last night in their televised reply to President Reagan's appeal for support for his tax increase.

House Majority Whip Thomas S. Foley (D-Wash.) endorsed the measure as a "critical first step" toward economic recovery. Sen. David L. Boren (D-Okla.) said it would not cure the deficits or the recession and claimed the mix of "good and bad provisions" is so even, "each member of Congress will have to strike the balance" for himself.

Foley and Boren were chosen by the House and Senate Democratic leadership to share the reply time made available by two networks after Reagan's speech. After disclaiming partisanship, they went off in separate directions.

Foley said, "I support the revenue bill because it is fair and because it brings some long-overdue moderation to the administration's economic program."

Boren, who voted against the Senate version of the bill last month, said it has "some good features," but "it certainly is not going to end the recession and put people back to work." Even if it is passed, he said, "deficits may still run as high as $450 billion" in the next three years.

The Oklahoma Democrat, who supported last year's Reagan tax-cut package, showed viewers a chart of the increase in unemployment since Reagan became president, and claimed that the Republican Senate had rejected Democratic efforts to make the tax increase "fairer."

He said the real answer to the recession lies in a bill co-sponsored by 33 Senate Democrats that would direct the Federal Reserve Board to lower interest rates.

Foley made no mention of that legislation, which is given little chance of passage this year, and said simply that "the president's position is right. He said we need this revenue measure -- and we do."

Foley, who opposed the original Reagan tax cut, said, "The great advantage of the tax bill now before the Congress is that it helps to balance many of the excesses enacted last year.

"It helps to cut down on the deficits -- and therefore helps relieve pressure on interest rates. It helps to cut down on the basic unfairness in last year's tax bill. It plugs some loopholes and helps to divide the cost of our society more fairly."

While Boren emphasized provisions the Senate Democrats tried unsuccessfully to change, Foley argued that "the Democratic influence on the compromise now before us is unmistakable. We won extended unemployment benefits. We helped shift the burden from the individual taxpayer. We have worked to protect the benefits of those who need that protection most."

Calling it a "vote for economic reality," Foley said: "It is in that spirit that I join with the president in asking for your support."

While the vote is expected to be close in both chambers, the strong support from the No. 3 House Democrat was particularly welcome to the administration, because more Democratic votes will be needed on that side of the Capitol than in the Senate.