Senate Republican leaders yesterday ducked a critical test vote on their own deficit-reduction plan for the second day in a row as a half-dozen rebellious GOP senators demanded concessions in exchange for their votes, including abandonment of cutbacks in Social Security benefits.

Faced with defeat if more than two Republicans defect, Majority Leader Robert J. Dole (R-Kan.) put off the vote until next week in hopes of quelling the insurgency within his own ranks, only to run into trouble from the Democrats when he tried to reschedule the showdown for Tuesday.

Central to the whole stalemate was a proposal to limit cost-of-living increases for Social Security, which both Democrats and dissident Republicans want to take the lead in opposing.

Sen. Paula Hawkins (R-Fla.) served notice she would propose restoring the full inflation adjustment for Social Security. But when Dole sought to reschedule the vote for Tuesday, Democrats objected unless they got first crack at trying to amend the package, presumably with a proposal for restoration of the full Social Security adjustment.

The GOP rebels include conservatives as well as liberals, ranging from Hawkins and Sen. Alfonse M. D'Amato (N.Y.) on the right to Sens. Charles McC. Mathias (Md.) and Lowell P. Weicker Jr. (Conn.) on the left. The issues range from Social Security for Hawkins and D'Amato to repeal of tax indexing for Mathias and a variety of social welfare programs for Weicker.

After Dole got apparently inconclusive results in a meeting with the dissidents, President Reagan invited two of them, Hawkins and D'Amato, to the White House for some further persuasion.

D'Amato said after the White House meeting that he was still holding out for some way to restore the full Social Security increase but indicated cautious optimism that an accord could be reached. "I'm hopeful because they need our votes," he said bluntly.

There apparently were some concessions in the works. Sources said, for instance, that Weicker has been assured $220 million more for programs for the handicapped, although some of his bigger demands, including $1 billion more for health programs, have not been met.

Dole has indicated he is prepared to accept a loss if it is close enough that he can claim substantial support for the plan on its initial test, which is largely symbolic but has been touted by Dole as a demonstration of will on the issue.

But the Republican rebels are banking on Dole's taste for victory ("I prefer winning," he acknowledges) to force concessions that neither he nor Reagan have been willing to make.

The Reagan-blessed plan would cut deeply into domestic spending and restrain Reagan's military buildup, for total savings of nearly $300 billion over the next three years, in hopes of cutting annual deficits by half to less than $100 billion by fiscal 1988. Social Security cost-of-living increases would be cut by about half, assuming modest inflation during the period.

As the Republican impasse continued, Democrats began to push for alternatives of their own, including spending freezes and some tax increases, although not on personal income. Minority Leader Robert C. Byrd (D-W.Va.) indicated he will have an alternative that will include a freeze in most domestic spending but the full inflation adjustment for Social Security. It reportedly will include tax increases.