Some conservatives, anxious to protect President Reagan's tax cutting program, are contending that this year's deficit may be $25 billion to $35 billion lower than administration predictions and are charging some of the president's economic advisers with a conspiracy of silence on the issue.

Both Treasury and the Office of Management and Budget staff dispute the conservatives' claim about the deficit, saying it is based on a mistaken assumption that trends in revenues and outlays in the first five months of this fiscal year would continue through the rest of the year. Other budget analysts agree with the official view that this year's deficit is likely to exceed $100 billion.

There are various technical reasons to explain the better-than-expected deficit numbers for the first five months of this year, officials say. Indeed, some analysts believe that the deficit may come in above the official $100.5 billion estimate published earlier this month.

Nevertheless, Rep. Clarence J. Brown (R-Ohio) said yesterday in an open letter to President Reagan, "Growing evidence suggests that the Office of Management and Budget estimates for the fiscal 1982 budget may be overstated by as much as $30 billion."

The letter, published in The Wall Street Journal yesterday, suggested that OMB had deliberately inflated the deficit estimates to push Congress into a budget compromise that could threaten Reagan's tax-cutting program. "I can't help wondering whether your economic agenda is once again being undermined by doubts about your program within your own administration," Brown wrote, in a reference to OMB Director David Stockman's controversial interview with Atlantic Monthly magazine last year.

The White House was told yesterday that "Treasury and OMB staff stand behind the re-estimates" of the deficit published earlier this month, an official said. Treasury Secretary Donald T. Regan said through a spokesman there is "just nothing to it," when asked about the lower deficit figures.

The flap began some weeks ago when Treasury reports showed the deficit for the first months of the fiscal year to be lower than expected. Revenues in the first five months were running 13 percent above the 1981 levels, while outlays were 10 percent higher.

A supply-side advocate in Treasury reportedly circulated an analysis based on these numbers to defend the president's tax program against other officials who are mainly concerned with reducing budget deficits.

"I wouldn't say the deficit numbers are false . . . Stockman can definitely make a case for his numbers. . . and I can make a case for mine," this official said.

Heritage Foundation, a conservative think tank, published a release recently arguing that if this trend of revenues and outlays continued for the next seven months of the fiscal year, revenues "(even allowing for the $9 billion to be lost through the July tax cut) could easily exceed $660 billion, leaving a projected deficit of around $65 billion."

James Capra, senior economist of the Federal Reserve Bank of New York, said yesterday however that he expected revenues for the last seven months of the fiscal year to show almost no growth from those in the same period in fiscal 1981. This would leave total 1982 revenues closer to $630 billion, or $30 billion lower than the Heritage numbers.

* Corporate tax payments in coming months will reflect the final payments companies make on their 1981 liabilities, and since company profits in 1981 were lower than in 1980, these tax payments will likely be lower, Capra said. In addition, the business tax cut enacted last year will cut corporate liabilities for 1981 and 1982.

* Last year Reagan's decontrol of oil prices led to a surge in windfall profits tax receipts. Sagging oil prices this year mean that the receipts from windfall profits tax by contrast are getting a little smaller than estimated each month, he said.

* Some tax changes designed to speed up corporate tax payments, such as increasing the interest due on late payments, may have succeeded in accelerating payments, Capra said.

* Tax cuts on interest and capital gains tax in last summer's tax bill may lower this April's tax payments on non-wage income, he said.

* A weaker economy in coming months and lower-than-predicted inflation may further reduce receipts, analysts suggested.

Outlays are similarly expected to pick up in coming months, an OMB spokesman said.

* Six monthly payments of Supplemental Security Income and Veterans benefits were paid in the first five months of 1981, and only five in the same months this year,

* In January 1981 the government paid a $2.1 billion Penn Central settlement, much larger than any comparable payments in 1982.