WASHINGTON (Reuters) - U.S. House of Representatives Democrats on Tuesday canceled a planned floor vote on legislation to set federal spending levels for next year after the party’s left wing demanded more money for domestic programs.

It was an awkward moment for Democrats, who won control of the House from Republicans in last November’s congressional elections, but now appear unable to unify liberals and moderates around a budget plan. The House vote had been planned for Wednesday and would have been the first step in negotiations with the Republican-led Senate over spending levels.

Republicans, when they led the House, had similar difficulties. Over the years, Congress has frequently failed to pass a budget blueprint.

It was only the opening salvo in what could be a long-running battle over spending levels and priorities, especially given Republican President Donald Trump’s calls for deep spending cuts to non-defense programs and differences over border security policy.

Such disagreements can sometimes lead to standoffs like the one that resulted in a 35-day partial government shutdown that ended earlier this year.

House liberals were unhappy because the pending Democratic legislation, based on a plan approved by the Budget Committee, had set spending caps for fiscal-year 2020 with $664 billion in defense spending, higher than domestic discretionary spending at $631 billion.

The liberals sought $33 billion more for domestic spending. But if that were added in, there was a danger not enough moderate Democrats would back the legislation, aides and lawmakers said.

“We fundamentally disagree with leadership. ... It’s the first time Democrats are back in control, that was our opportunity to put parity between defense and non-defense,” said Representative Mark Pocan, a Democrat and chairman of the Congressional Progressive Caucus.

“Education, infrastructure and healthcare are really the issues that we ran on last November, and we’re just trying to get the (Democratic) caucus to do that,” Pocan said.

Democratic leaders said talks with the Senate could still go ahead and they inserted provisions in another measure that passed on Tuesday allowing House appropriators to start work on individual spending bills.

Democrats said House Speaker Nancy Pelosi would be meeting Republican Senate Majority Leader Mitch McConnell to try to reach a spending agreement.

“If we can do that, that’s the best way to do it, because then the Senate will agree to it,” said House Majority leader Steny Hoyer, adding that hopefully, McConnell could then get the White House to agree.