President Carter may have sent Congress a time bomb in the combination tax reduction and tax "reform" package he unveiled last weekend.

Although tax experts generally are praising the President's new proposal as measured and wellcrafted, the package is wired so dlicately that many observers are fearful it easily could blow up. Indeed, some say the way it's put together all but invides touching off legislative explosion.

The danger comes because the package hinges so heavily upon Congress' approval of a half-dozen key elements that most long-time veterans believe aren't likely to pass. IF the lawmakers reject any of these, it could open the package up to major rejuggling.

Moreover, initial reaction from the House Ways and Means Committee, which will be first to consider the tax package, indicates the lawmakers may be heading in just such a direction. Even conservatives are worried that the proposal might blow apart.

There are these dangers:

The packages relatively low cost depends on companion enactment of a spate of controversial "tax reform" proposals that the lawmakers aren't likely to pass -- a prospect that threatens to raise the overall price [WORD ILLEGIBLE] and bloat the already large federal budget deflcit.

Although Carter proposed $24.5 billion in "net" tax cuts the package naturally comprises $33.9 billion [LINE ILLEGIBLE] by $9.4 billion in "reforms" and other revenue-raising measures. If the lawmakers reject these revenue gainers, the cost could rise substantially.

Yet, initial surveys of the House Ways and Means Committee indicate the Presidents likely to win approval of only a handful of these "reforms." Carter has said he'll seek to trims the tax package if the reforms don't pass, but isn't likely to succeed. Many lawmakers want even larger cuts.

The resulting [WORD ILLEGIBLE] is likely to open the way for individual senators and congressmen to load the bill down the dozens of proposals for special-interest tax breaks - turning it from a loophole, closing measure into a traditional congressional "Christmas tree hill" laden with tax goodies for everyone.

The Ways and Means Committee alone already has 11 pending proposals on its agenda - including special tax breaks for the ailing steel industry, a conservative bid to end restrictions on the carry over of capital gains, and postponement of the 1976 crackdown on tax treatment of Americans living abroad.

Another potentially costly rider is the college tuition tax credit now being pushed by Senate Republicans. Depending on which version is enacted, that could increase the tax bill's cost by $1.3 billion to $4.5 billion a year - and the administration may not be able to head it off, despite a last-ditch effort.

In addition, the Ways and Means schedule includes several billion dollar's worth of other pending tax legislation. Some samples: Expanding the "earned income credit" as part of the new welfare program, extending the scope of individual retirement accounts, and revamping last year's jobs credit.

The dangers may be heightened by the fact that, for the majority of lawmakers, this is an election year -- a factor that in the past seems to have brought out the worst in Congress at least as far as tax legislation is concerned.

Election-year pressures traditionaly spur the lawmakers to Ment the size of tax reductions and add on more new tax breaks than ever for special-interest groups. Until now, congressional leaders have been loathe even to consider a tax bill before an election.

The process will be complicated by a sharply-compressed legislative timetable -- the result [PARAGRAPHS ILLEGIBLE]