WASHINGTON, Aug 23 (Reuters) - Democrats faced a test of unity in the U.S. House of Representatives on Monday as they prepared for a possible vote on President Joe Biden"s sweeping plan to expand spending on social programs, even though it was not clear if there was enough support to pass it. The measure, on which talks continue, sets the broad outlines for $3.5 trillion in spending on education, childcare, healthcare and climate measures favored by Biden and pays for them with tax hikes on the wealthy and corporations. But moderate Democrats have threatened to vote against the plan, saying the House should first pass a $1 trillion infrastructure bill that has already won approval from Democrats and Republicans in the Senate. Both measures are priorities for Biden. That could potentially scuttle the spending plan in the House, where Democrats hold a 220-212 majority. No Republicans are expected to support it. The House went into recess on Monday evening, leaving up in the air how soon the vote would take place. Representative Josh Gottheimer, the leader of the group of Democratic moderates who want a quick vote on the Senate-passed bipartisan infrastructure bill, was meeting with House leaders behind closed doors. As the talks continued, House Speaker Nancy Pelosi told reporters: “We’re legislating. So we’ll let you know when we’re finished legislating.“ Asked if leaders had the votes to pass their plan, she said: "When we bring the bill to the floor, we will." Pelosi has sided with liberals who worry they might lose leverage on the social-spending effort if they first pass the infrastructure bill. "We cannot squander this majority and this Democratic White House by not passing what we need to do," Pelosi told Democrats in a meeting before the vote. Monday"s planned vote would pass the budget plan and clear the way for the House to vote on the infrastructure bill, as well as a separate voting-rights measure. The House could pass the voting-rights bill as soon as Tuesday, but it is not clear when the infrastructure bill would come up for a vote. The House vote would allow Democrats to pass the social-spending measures on a simple majority vote in the Senate, rather than the 60 votes required for most legislation in that chamber. The Senate is split 50-50 between Republicans and Democrats. Democratic Vice President Kamala Harris would cast the tie-breaking vote. Reporting by Susan Cornwell and David Morgan; Editing by Andy Sullivan and Peter Cooney Our Standards: The Thomson Reuters Trust Principles.