Like newly reconciled lovers, Democratic Party and AFL-CIO leaders reminisced today about the good times they had together and vowed never to break up again, if at all possible.

"It is great to be back together again," said House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), addressing the federation's 14th constitutional convention here. "Some of you are old enough to remember the 1930s and when, together, we made America great.

"Together, the Democrats and labor made a middle-class America. We put together 30 of the greatest, fruitful and beneficial years that a democracy ever had. We did it together, labor and the Democrats working together. I say I welcome you to the fold," O'Neill said in a reference to the federation's plan to increase its presence and participation in the Democratic Party.

Under its late president, George Meany, who believed that the federation should not be the "captive" of any political organization, the AFL-CIO played an ostensibly neutral role, although it frequently endorsed Democratic candidates.

But strains developed under former president Jimmy Carter, and the cracks in the relationship were reflected in the 44 percent vote union families gave to Republican Ronald Reagan in 1980.

The new romance was short-lived. Reagan, in the thinking of many labor officials, has turned out to be antiunion, and AFL-CIO President Lane Kirkland has argued successfully that the current state of affairs is largely a result of the federation's institutional reluctance to take political sides.

Now there are 15 AFL-CIO representatives on the Democratic National Committee and five others serve on the party's executive committee. According to federation sources here, the DNC also plans to establish a formal labor committee, with two AFL-CIO members acting as cochairmen, in the next few weeks.

In a break with tradition that underscored its change of heart, the federation's leadership did not invite President Reagan, Vice President Bush or Labor Secretary Raymond J. Donovan to address its biennial meeting.

O'Neill, who spent much of his time here today excoriating the president's economic policies, clearly was happy about his party's reunion with labor. Many of the 900 delegates seemed similarly pleased, giving O'Neill a standing ovation when he entered the convention hall and another as he left.

Convention delegates later approved a policy statement proposing measures to counter the nation's economic woes. Among other things, the proposal calls for the restoration of several federal jobs programs, including public service employment; restrictions on imports, and targeted tax cuts to help industries retool.

Kirkland, meanwhile, praised O'Neill's "courage and political life" and called him the "chief antagonist of those who would strip away the programs" labor has built up over the years.

But like many lovers taking another chance, Kirkland's renewed affection was laced with caution. He asked the speaker to remind his colleages on Capitol Hill, particularly the self-styled "boll weevils"--conservative Democrats who have backed the administration in its budget proposals--that they ought to "stand and fight in the interest of the concerns of the plain working people of this country."

In a related development today, several AFL-CIO executive council members said in interviews that they will not accept a White House invitation to meet Reagan Dec. 2.

"I am not going under any circumstances. I won't be used as window dressing for a slick public relations campaign," said Glenn Watts, president of the Communications Workers of America. Others on the council, however, among them Joyce Miller, vice president of the Amalgamated Clothing and Textile Workers Union and the council's only female member, said they would attend.

Meanwhile, among more than 200 resolutions before the convention are proposals to raise affiliate dues by about $14 million annually by 1983 and to express support for striking air traffic controllers.

About 30 black delegates are pushing for two of five vacant Executive Council seats to be filled by black union leaders. Hispanic delegates are expected to ask that one of the seats go to a Hispanic union leader.