Democrats drew first blood yesterday on President Reagan's fiscal 1987 budget, as many Republicans went on the defensive. But by November, some political experts speculate, the new world of Gramm-Rudman-Hollings may look risky to incumbents of both parties.

The prospective opponents in South Dakota's Senate race -- expected to be one of the toughest battles in the struggle for control of that body -- teed off within moments of the budget release, but not on each other.

Rep. Thomas A. Daschle, the Democratic challenger, said, "The president is sending a clear message to average Americans that he would trade their futures for a wasteful, excessive defense buildup." Daschle told his farmers that Reagan proposes cutting farm support programs "by over 25 percent," and his city residents that Reagan would "cut the housing department's budget by 47 percent." Veterans heard that Daschle felt "outrage and amazement" that the president would propose ending the newly enacted GI bill.

In the face of this assault, Sen. James Abdnor (R) said that "White House budget planners are out of touch with reality if they believe they can sell Congress on huge increases in defense spending while calling for deep slashes in agriculture, transportation and education programs . . . . If the administration is looking for congressional support for this budget, they had better not knock on my door . . . . As far as I'm concerned, this budget is not acceptable by any stretch of the imagination."

The kind of posturing that Daschle and Abdnor were doing -- positioning themselves as protectors of popular programs and as critics of other, "wasteful" spending -- is a native reflex for almost anyone on Capitol Hill and is standard for members of Congress in any year.

But this year is different because the Gramm-Rudman-Hollings balanced-budget law holds out the threat of automatic cuts if Congress and the president fail to agree on some $40 billion of reductions and/or new revenues by September.

It is that prospect -- staring incumbents of both parties in the face -- that causes some political consultants to say that this could be a rough election for officeholders of both parties.

"The political shields of the past are no longer in place," Richard B. Wirthlin, the president's pollster, said. "(Incumbent) Republicans and Democrats are going to have to take what could be a rather uncomfortable walk in the sun."

Joe Rothstein, the consultant who produced Tuesday's Democratic response to the president's State of the Union message, agreed. "My guess is that incumbents generally will have tougher political ground to defend," he said, "because Gramm-Rudman looks like an abandonment of responsibility to make the hard decisions . . . like passing the buck. I would think any challenger would have the opportunity to run against an incumbent, citing cuts in popular programs at home and saying that if the incumbent had done the right thing, it would not have happened."

That may be the situation come autumn, but in this winter of budget discontent the Republicans are pleading almost plaintively for bipartisanship while the Democrats are making plans to parade the Reagan budget around the country as a warning to those who might be contemplating voting for the GOP.

House Budget Committee Chairman William H. Gray III (D-Pa.) said his panel will begin a cross-country set of hearings on Reagan's proposed cuts in five major cities on Monday.

Gray indicated that he will bring the original Reagan budget out of his committee next month and force Republicans to vote with or against the president.

Gray said that after the Reagan budget has been defeated, he will attempt to forge a Democratic version, using Reagan's own optimistic economic assumptions, that reaches the Gramm-Rudman-Hollings target of a fiscal 1987 deficit of $144 billion without a tax increase. In the process, said Rep. Martin O. Sabo (D-Minn.), "we will certainly scale back defense and bolster the domestic programs."

Senate Republicans are more likely to include a tax increase in their April package, as they did last year. But the crunch will come in August, when there is widespread expectation that the neutral estimates of the three main budget agencies will show that the April congressional package falls short of the $144 billion goal.

If no compromise can be forged at that point among Reagan, the Senate and the House, the "trigger" will be pulled and all unprotected programs -- defense and domestic -- will be cut across the board.

That is a prospect that scares incumbents of both parties. Rep. James R. Jones (D-Okla.), a former Budget Committee chairman who is an unannounced Senate candidate, is plumping for an early "bipartisan summit" on the budget. "I don't think Democrats can take comfort next October by saying there was nothing we could do to stop Gramm-Rudman. People elect folks to do something," he said.

In neighboring Louisiana, Rep. W. Henson Moore, the Republican candidate for an open Senate seat, is hedging no bets. His reaction to the Reagan budget is "right on, full speed ahead . . . . I will vote for any package that achieves the Gramm-Rudman target without a tax increase. My people understand there will be some revenues lost to Louisiana, but they know that balancing the budget and reducing interest rates is worth more to us than any government program."

Some consultants think the threat to incumbents is exaggerated. Democrat Franks said "people rarely blame their own congressman" for the institutional failures of Congress. Republican pollster Robert Teeter said he had never known large numbers of incumbents to lose in a time of general prosperity.

But Democratic consultant Robert D. Squier said, "If you're a governor or legislator in a state that balances its budget, all you have to do is say [of the incumbent member of Congress], 'You people are so incompetent that you have to lock the door against yourselves and then you threw away the key.'