The Washington business lobbying community, once expected to oppose the Senate Finance Committee's radical tax-overhaul bill, is rapidly falling in line to support it.

The U.S. Chamber of Commerce is likely to endorse the far-reaching package as early as today, with the National Association of Manufacturers and the Business Roundtable expected to follow suit soon.

All three opposed the tax-revision bill approved by the House last year. But the Finance Committee's bill, with a top individual tax rate of 27 percent and a smaller tax increase on corporations, has changed the political landscape.

The shift in attitude is illustrated by the experience of Sens. David H. Pryor (D-Ark.) and Rudy Boschwitz (R-Minn.) when they addressed a convention of 1,500 machine manufacturers this week.

The senators were braced for a torrent of opposition, and the manufacturers did spell out numerous objections to the measure, which would raise corporate taxes by $108 billion over the next five years.

In addition, Pryor said, virtually all 1,500 of them raised their hands when asked who had an Individual Retirement Account. The bill would curtail the deductions for IRAs.

"Then I asked, 'Suppose you had to vote yes or no on the whole bill at 4 p.m. today?' " Pryor said. "About 1,450 raised their hands to say 'yes.' I was amazed. Absolutely amazed."

Some business groups have supported tax revision from the start. But the fact that the largest business lobbies now appear ready to support the bill shows the momentum it has developed.

President Reagan has embraced it. And as members of Congress packed up for their Memorial Day recess this week, he vowed to lobby against amendments, even those to maintain current IRA deductions.

The plan to slash personal tax rates from a maximum of 50 percent to 27 percent and the corporate rate from 46 to 33 percent appears to have overwhelmed business objections to the repeal of many deductions and credits.

The American Bankers Association endorsed the bill Tuesday, and several insurance groups say they favor the plan despite the cutbacks it proposes for their tax breaks. A broad-based coalition of labor, consumer and business groups supporting the bill is said to be adding members daily.

Even organizations nominally opposed to key provisions of the Finance Committee package may harbor some closet supporters of the bill. In meetings with senators this week and last, members of the two most outspoken critics -- the National Association of Realtors and the National Association of Homebuilders -- frequently confided that the 27 percent top tax rate for individuals looks very appealing.

"I find most of the home builders, if you ask the outright question of [whether they would prefer] present law or the committee bill, say they'll take the committee bill," said Sen. Lloyd Bentsen (D-Tex.). "They are not responding with all the alarm their hired guns thought they would."

Sen. John C. Danforth (R-Mo.) said his conversations with industry opponents generally end the same way: "They say, 'Here is our group position.' And then when they sidle up at the end of the meeting, the comment whispered in your ear is, 'Hey, it's a pretty good bill.' "

Pryor said many business people who he thought would oppose the bill have wound up supporting it or lying low. "I saw a real estate syndicator from California the other day, and I said, 'Oh my gosh, let me get my bullet-proof vest.' But he said, 'Not so fast. I came to thank you for voting for the bill.'

"Then, take the auto dealers. I thought they would march on Washington. But I've had only two or three calls. They said that with that 27 percent rate, heck, people will have more money to buy cars. Now the restaurant people are real concerned, but none of this is as intense as I had thought it would be."

Organized lobbying by financial institutions to preserve IRAs also has been lighter than expected, but senators say they are hearing plenty from back home. Sen. Daniel Patrick Moynihan (D-N.Y.) has received 1,500 letters a week asking for retention of the full IRA deduction -- a level described by an aide as "in the pay-attention category."

Other senators' offices are receiving IRA letters by the hundreds. A letter from a banker urging Sen. Russell B. Long (D-La.) to run for governor ended: "P.S. We support your tax bill but would like to retain the present IRA status."

Under the bill, workers covered by any private pension plan could no longer deduct deposits into IRA accounts, now permitted for up to $2,000 per year. Interest earned on those accounts would remain tax-free until withdrawn at retirement. Those without pension coverage would retain the full IRA deduction.

Reagan told members of the Senate Finance Committee Tuesday that lower tax rates would offset the partial loss of the IRA for many taxpayers.

Business lobby groups say similar tradeoffs have stifled their opposition to the measure. But some said they are simply holding their fire until the House-Senate conference committee meets to reconcile differences between the two bills.