By Richard Cowan and Susan Cornwell

WASHINGTON (Reuters) -The U.S. House of Representatives on Tuesday was on the verge of breaking a deadlock over increasing the federal government's $28.9 trillion debt limit, in hopes of avoiding an unprecedented default on payments.

A House vote on passage of a measure making it easier for the Senate to promptly raise the debt limit with a simple majority vote of its 100 members was expected imminently.

"Increasing the debt ceiling will prevent us from defaulting on debt we already owe. It's about investments that the Congress previously approved," Democratic Representative Steven Horsford said during House debate.

Earlier in the day, House Speaker Nancy Pelosi warned of dire consequences without fast action by Congress.

"We must address the debt limit to stave off an unnecessary and catastrophic drop of trillions of dollars of GDP and a devastating downgrade to our credit rating," Pelosi said in a statement.

But House Republicans, who were expected to oppose the measure in lock-step, argued against any action, knowing Democrats intended to shoulder the burden on their own under a deal privately negotiated by both parties.

"Make no mistake. This debt ceiling is being lifted to pay for trillions of wasteful socialist spending," argued Republican Representative Kevin Brady.

He was referring in part to President Joe Biden's $1.75 trillion "Build Back Better" domestic investment initiative that Democrats hope to pass in the Senate this month, arguing that it is self-financing and thus would not significantly add to the debt.

Democrats say the increased borrowing authority is needed largely to cover the cost of tax cuts and spending programs during former Republican President Donald Trump's administration, which congressional Republicans supported.

' BEST (NYSE:BEST) INTEREST OF THE COUNTRY'

Normally, most legislation needs support of at least 60 senators to clear procedural hurdles. Senate Republican Leader Mitch McConnell sent a clear message that his party will help facilitate a debt limit increase even though its members aim to vote against the actual bill implementing it.

"I think this is in the best interest of the country by avoiding default," McConnell told reporters earlier in the day, adding, "We'll be voting on it Thursday."

The dollar amount for the proposed new statutory debt limit still must be determined, but it was expected to be enough to carry the Treasury Department through next November's congressional elections, according to Democratic Senator Elizabeth Warren.

The breakthrough strategy, brokered by Democratic and Republican congressional leaders following months of political infighting, would establish a two-step approach for raising the Treasury Department's borrowing authority.

First, both chambers would vote on the bill on the expedited Senate procedures. Senate passage of that bill will need a supermajority of at least 60 and thus requires Republican cooperation.

A second bill actually raising the debt limit under the expedited procedure would then be debated for a maximum of 10 hours in the Senate, instead of the open-ended debate that can delay or kill many Senate bills.

Senate passage would clear the way for a final vote by the House.

The expedited procedure for the debt limit was included in a bill to postpone Medicare cuts that would otherwise take place starting Jan. 1. That is a bill most lawmakers would not want to delay or kill.

By facilitating passage of a debt limit increase but not actually voting to implement one, Republicans would give themselves ammunition to attack Democrats in the 2022 congressional election campaigns for raising the $28.9 trillion debt limit.