President Carter, still trailing at the midway point of his reelection campaign, stepped up his assault on Ronald Reagan today, sharply criticizing the Republican nominee by name on his arms control policy and a number of other issues.

Speaking at a backyard meeting in an affluent suburb northwest of here, the president called Reagan's nuclear arms policy "very dangerous" and said it demonstrated "Gov. Reagan's lack of understanding of these issues."

"If you've got just a strong military and you are jingoistic in spirit and just want to push people around and show the macho of the United States, that is an excellent way to lead our country toward war," Carter said. "You've got to have a strong military, and you've got to have arms control and you've got to have a stable, sound policy that is well understood."

Earlier today, speaking in the Milwaukee area, the president sought to assail Reagan on his proposal to transfer some federal programs to state and local governments. But the confused effort did not come off as planned, and so by the time Carter reached the traditionally Republican stronghold of Du-Page County, Ill., he reverted to his familiar suggestion that Reagan election would increase the risk of war.

What was striking about the attack was its stridency and directness. Although Carter has made the "war and peace" theme the centerpiece of his campaign, he has usually done so in a low-keyed manner, often not even mentioning Reagan by name.

However, over the weekend a number of news organizations, including The Washington Post, published mid-campaign assessments showing Reagan holding a steady lead, particularly in projected number of electoral votes. In light of that, Carter strategists apparently have decided to step up their attacks on the GOP nominee, particularly on foreign policy, in the hope of injecting some new movement into the president's campaign.

Reagan has called for scrapping of the pending strategic arms limitation treaty with the Soviet Union and a return to negotiations while the U.S. beefs up its nuclear arsenal.

Speaking to about 50 people in a backyard of a home in suburban Addison township, Carter hit hard on this issue and on the "war and peace" theme generally.

Citing previous Reagan statements suggesting American military involvement abroad, the president said:

"The Oval Office is not a place for simplistic answers. It is not a place for shooting from the hip. It is not a place for snap judgments that might have serious consequences."

From the suburbs, Carter journeyed to Daley Plaza in downtown Chicago where, joined by Mayor Jane Byrne, he addressed a voter registration rally. Tonight, the president spoke to a Democratic fund-raising dinner here before returning to Washington.

Carter grew even more strident at the fund-raising dinner tonight, suggesting that a Reagan victory would be "a catastrophe" that might result in the alienation of "black and white" and "Christian from Jew."

"This is my last campaign, the last political race that I will ever run," the president said. "I do not intend for it to end by turning over the government of the United States to people whose political philosophy and views about this country are directly contrary to everything I believe with all my heart and soul." He added that the prospect of Reagan's election is "too bleak to contemplate."

Earlier, in a speech at a technical college in the working-class Milwaukee suburb of West Allis, Carter sought to rekindle the controversy over Reagan's so-called "$90 billion plan" of 1976.But Reagan has modified his position on the issue of transferring some federal programs to state and local governements since then, a fact that the president was forced to acknowledge by a change in the prepared text of his speech.

The confusing and highly technical criticism of the Reagan proposal, delivered in a flat, low-keyed style by Carter, left his audience at the Milwaukee-area technical college unmoved.There was only silence from the crowd as the president attacked the transfer plan and charged that Reagan's call for a major federal income tax reduction "is like quicksilver -- it glitters, it promises quick results, easy answers, but it ends up being worthless."

The dispute over the transfer plan centers on Reagan's 1976 proposal that some federal programs such as welfare be transferred to state and local government control and financing. He put a $90 billion price tag on the plan, but argued that the higher state and local taxes needed to pay for the program would be offset for taxpayers by equal reductions in their federal income taxes.

President Ford used the "$90 billion plan" with devastating effectiveness against Reagan in the 1976 GOP primary in New Hamsphire. Reagan quickly backed down on the proposal and has since maintained that he would allow states to retain enough in federal tax collections to pay for the programs returned to their control.