By David Morgan and Richard Cowan

WASHINGTON (Reuters) -More than 20 U.S. House Republican hardliners warned Speaker Kevin McCarthy on Monday that they will try to block their party's fiscal 2024 appropriations bills unless spending levels are cut below levels that McCarthy and Democratic President Joe Biden agreed to in May.

The hardliners, including members of the House Freedom Caucus, also called on McCarthy to delay appropriations votes in the House of Representatives until all 12 government funding bills have been finalized and can be subjected to a side-by-side review.

The threat comes in the face of a looming showdown between the Republican-controlled House and Democratic-led Senate, potentially complicating efforts to avoid a government shutdown after the current fiscal year ends on Sept. 30.

"We expect all appropriations measures ... to be in line with the enacted FY 2022 topline level of $1.471 trillion," the 21 lawmakers said in a letter, led by Representatives Scott Perry and Chip Roy, both prominent House Freedom Caucus members.

"Absent adhering to the $1.471 trillion spending level ... we see an impossible path to reach 218 Republican votes on appropriations or other measures," the letter said.

The group includes lawmakers who shut the House floor down last month to protest McCarthy's May deal with Biden that averted default on U.S. debt.

McCarthy's office did not respond to a Reuters query seeking comment on the letter.

In the meantime, Senate appropriators are aiming for bipartisan deals -- all of which point to difficult negotiations ahead -- as Congress returned on Monday from the two-week July 4 recess.

A host of hot-button issues ranging from abortion to transgender rights are expected to be pulled into upcoming debates, further complicating matters. If lawmakers fail to agree on a budget before the next fiscal year begins on Oct. 1, the United States could see its fourth partial government shutdown in a decade.

Senate negotiators, who were largely sidelined during the recent talks between House Republicans and the White House over the federal government's $31.4 trillion debt ceiling, were working on bills that are attracting strong bipartisan support.

"We are determined to continue working together in a bipartisan manner to craft serious funding bills that can be signed into law," Democratic Senator Patty Murray and Republican Senator Susan Collins said in a joint statement.

Republicans hold the House by a narrow 222-212 majority, while Democrats hold a razor-thin 51-49 majority in the Senate, meaning that nothing can pass into law without votes from both parties.

CONFLICTING TARGETS

Leaders of the two chambers don't even agree on the spending targets they are aiming at.

Senate negotiators plan to hold to the $1.59 trillion discretionary spending target that Biden and McCarthy agreed to in the May deal.

House Republicans last month voted on a lower target of $1.47 trillion, which would cut spending for the environment, public assistance and foreign aid.

The House target does not take into account $115 billion in existing funding that Republican leaders could redirect to party priorities to compensate for cuts.

House Republicans are also trying to use the legislation to rescind key Biden priorities in areas such as climate change and tax collection. They also seek to eliminate or alter some existing programs involving workforce diversity, transgender protections and women's access to abortion that Democrats are fighting for.

"I am ready to end this charade of considering extreme Republican funding bills and join my colleagues in both chambers and on both sides of the aisle in working toward a final agreement" on government spending for next year, Democratic Representative Rosa DeLauro said in a statement on Friday.

DeLauro, the senior Democrat on the House Appropriations Committee, noted that House Republicans "know and have said publicly, that in the end they are going to need Democratic votes to keep the government open."

Failure to agree on appropriations could lead to a partial government shutdown into the autumn and winter that could hobble many federal activities, including air traffic control, military pay increases and the operation of national parks.

Representative David Joyce, who chairs the Republican Governance Group, or RG2, a more moderate group of 42 lawmakers concerned with House governance, said there could be scope for a short-term funding deal to maintain government operations while talks continue into the fall.

"I'm not a big fan at all of shutdowns, and I don't think anybody in RG2 or our groups are really thinking about that," Joyce told Reuters. "We're trying to think how to make things work."

(Reporting by David Morgan and Richard Cowan; Editing by Scott Malone, Jonathan Oatis and Leslie Adler)