President Reagan, portraying himself as newly determined to drive down unemployment, yesterday closed out a year-long dispute with Congress and signed into law a job training bill that the White House said would serve a million Americans a year.

Democrats quickly attacked the signing ceremony as a political ploy, saying Reagan had embraced the bill only as the elections approached.

The bill "does not create a single new job," said House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.). "People want jobs, not ceremonies."

"The record will show that this important job legislation was delayed for months in both the Senate and the House because of White House intransigence and repeated veto threats," added Sen. Edward M. Kennedy (D-Mass.), who sponsored the bill in its original form.

But Kenneth M. Duberstein, head of the White House office of legislative affairs, said, "To characterize it as something we had come lately to is not accurate." And an undaunted Reagan said of the Democrats: "That is campaign talk and they know it. It isn't true. Funny in an election year how careless the language gets."

The president said that "20 million American workers now rely on skills that won't be needed" in the future and that the bill will "train more than 1 million Americans a year in skills they can use." At the same time he said the new program, a replacement for the discredited Comprehensive Employment and Training Act, which Reagan knocked out of the budget last year, is "not another make-work, dead-end, bureaucratic boondoggle."

At Reagan's side as he signed the bill were Labor Secretary Raymond J. Donovan and about 20 young people that the White House said were trainees who had benefited from the existing federal job programs.

But no congressional sponsor was present. White House officials said this was because these congressional sponsors (who actually proposed the new program before any White House plan went to Capitol Hill) were absent because some Republicans like Sen. Dan Quayle (Ind.) could not come and it did not make sense to invite only Democratic sponsors like Kennedy and Rep. Augustus F. Hawkins (Calif.).

In signing, the president expressed thanks to Quayle, Kennedy, Sen. Orrin G. Hatch (R-Utah) and Reps. James M. Jeffords (R-Vt.), John N. Erlenborn (R-Ill.) and Hawkins for their work on the bill.

The bill kills all authority for the controversial public service jobs program that was the main element in CETA. At one point this part of CETA helped fund 725,000 jobs in state and local government agencies and helped boost total federal outlays on job programs over $10 billion. In recent years the public service employment program had fallen victim to charges of waste and politics, and at Reagan's request its funding had already been cut to virtually nothing last year.

However, the bill provides permanent authorization for the government to continue the major role in job training for disadvantaged youth and certain other groups that it has played since the Kennedy administration in 1962.

Although the final level of funding for the program is not yet set, the stopgap financing resolution now in effect for the whole government allows about $2.4 billion in fiscal 1983 (which will be a transition year to shift over from CETA) for the basic job-training program, plus $1.9 billion for related programs like summer youth jobs, the Job Corps, senior citizen jobs and the work incentive program.

Total projected spending for the new program and all these others thus comes to about $4.3 billion at the moment, about the same level as for fiscal 1982.

By contrast, the administration sought only $1.8 billion for the new program and an added $600 million for all the related programs combined, most of which it wanted to kill altogether.

The bill represents a compromise put together by Quayle, Kennedy, Hawkins, Jeffords and others on the congressional labor committees. Until virtually the last minute, according to both Republican and Democratic sources on Capitol Hill, the Labor Department held out the threat of a veto if the administration didn't get its way on certain items, and it wasn't until Quayle, Hatch, Jeffords and several other Republicans went to the White House for a special meeting Sept. 22 that the matter was settled.