(Adds credit rating downgrade from S&P in 3rd & 4th paragraphs)

By Tim McLaughlin

BOSTON, March 5 (Reuters) - A go-to lender for U.S. electric cooperatives has $4 billion in exposure to the Texas market, where last month’s deep freeze slammed the finances of several co-ops hit with astronomically high gas and electric prices during the state’s grid blackout.

The latest quarterly financial disclosure from the National Rural Utilities Cooperative Finance Corporation (CFC) shows the Texas market accounts for 15% of the lender’s $27.1 billion in outstanding loans. Dulles, Virginia-based CFC has not had any loan defaults in its electric utility loan portfolio since fiscal 2013.

On Friday, S&P Global Ratings said that it lowered its long-term credit rating on CFC to 'A-' from 'A', with a negative outlook.

The credit rating agency said Texas' main grid operator, the Electric Reliability Council of Texas (ERCOT), could exercise so-called uplift provisions. In that scenario, the unpaid bills of defaulting utilities, for example, would be absorbed by remaining participants and weaken their creditworthiness.

Numerous Texas electric co-ops are facing potential bankruptcy due to the massive bills incurred when power prices spiked during the Texas freeze that killed several dozen people and left millions without power for days.

CFC, which is searching for a replacement of its long-time chief executive, will be put to the test in the coming weeks and months as Texas electric utilities and cooperatives sort out billions of dollars in unpaid charges from the collapse of the state’s main electric grid.

On Monday, Brazos Electric Power Cooperative Inc, the biggest electric co-op in Texas, filed for bankruptcy protection in Houston, citing a disputed $1.8 billion debt to the state's main grid operator.

CFC has disclosed that it has $85 million in outstanding unsecured loans and letters of credit to Brazos. CFC said it is not currently able to assess to what extent, if any, the bankruptcy process may impair the lender’s ability to collect the amounts outstanding, according to a U.S. regulatory filing.

CFC did not return a message seeking comment.

S&P said it believes CFC has adequate funding and about $8.7 billion in liquidity to absorb losses.

CFC is a tax-exempt, member-owned finance cooperative run by CEO Sheldon Petersen, who has quadrupled the size of CFC’s loan portfolio over the past 25 years. Global executive search firm Korn Ferry is looking for someone to replace Petersen, who plans to retire this year.

About 98% of CFC’s portfolio consists of loans to rural electric systems. Its financing supplements the loan programs of the Rural Utilities Service (RUS) of the U.S. Department of Agriculture. CFC makes loans makes so electric co-ops can buy and build transmission lines and generation facilities.