The National Conservative Political Action Committee, once the kingpin of right-wing PACs, is deeply in debt at a time when the cash flow to conservative organizations has slowed to a trickle.

Further, according to two studies, NCPAC, which has said it spent more than $10 million on President Reagan's reelection campaign, apparently spent most of the money on millions of NCPAC fund-raising letters using Reagan's name.

According to the most recent reports filed at the Federal Election Commission (FEC), NCPAC was $4.2 million in the red at the first of the year, with the largest debts owed to Richard A. Viguerie's direct-mail company ($1.44 million) and to Response Graphics of Cleveland ($1.78 million).

A study by Michael Malbin of the $10.1 million spent by NCPAC on the 1984 presidential races showed that 85 percent of the cash was used for "mail services and printing," while only 8 percent went to advertising and other expenditures. The remainder went to payroll, office and other expenses.

"The FEC filing by NCPAC and another conservative group, RuffPAC look suspiciously like two PACs using Reagan's name for their own fund-raising and then reporting the activity as an independent expenditure," Malbin, resident fellow at the American Enterprise Institute, wrote. If so, "independent spending by the two largest PACs to engage in the activity was nothing more than a paper tiger."

Malbin's speculations were supported by a more detailed investigation by Ronald Brownstein of the National Journal.

Brownstein found that NCPAC had sent out 28 million letters through the Viguerie company, many of which read: "Do you want President Reagan to be reelected . . . ? A $15 contribution will let us mail over 42 letters to voters . . . , $25 will pay for air time to run one radio advertisement . . . , $500 will cover the full production costs of a 30-second commercial."

Instead, Brownstein said, the cash "was earmarked solely for additional mailings in search of further new donors." Brownstein quoted Leif E. Noren, NCPAC's executive director and treasurer, as acknowledging "that particular section of the mailing was misleading."

Noren refused to discuss the issue when contacted by The Washington Post. Michael Barnhart, a NCPAC spokesman, said he could not comment on the National Journal study until he had read it.

Viguerie and NCPAC have been fighting over the legitimacy of the debt owed The Viguerie Company, but Viguerie indicated in an interview that the conflict may be close to resolution.

"Terry Dolan the head of NCPAC and I had breakfast earlier this week," he said.

Viguerie refused to discuss in any detail the material in the Malbin and Brownstein studies, charging that the "liberal press" is biased against him and other conservatives. "Conservatives gave up long ago trying to promote conservatism through the liberal media," he said.