An analyst at BMO Capital Markets warned that mortgage finance giants Fannie Mae and Freddie Mac may write down $21 billion of tax-related assets if President Donald Trump keeps his vow of a deep cut in the federal corporate tax rate.

These assets, known as deferred tax assets, are items such as tax credits that may be used to reduce a company's taxes.

If the rate cut is lowered to 20 percent from 35 percent, the value of Fannie and Freddie's deferred tax assets is worth less and it would be recognized against their capital.

The two agencies, which guarantee home loans and mortgage-backed securities, are holding little capital since they are not allowed to retain their earnings after they have been under conservatorship or government guardianship due to heavy losses from the housing market collapse more than eight years ago.

Fannie drew $116.1 billion and Freddie $71.3 billion from the U.S. Treasury Department to cover those losses. They have remitted all their profits, which are more than their draw, to the Treasury under the conservatorship arrangement.

In absence of much capital cushion, the government-sponsored enterprises (GSEs) would need borrow nearly a total of $17 billion from Treasury, BMO's head of fixed-income strategy, Margaret Kerins, wrote in a research note.

Such a move, however, would not hurt the value of their bonds or disrupt mortgage market, she said.

"However, the potential for renewed draws is likely to be politically unpopular and may spark preemptive Treasury action and Congress to prioritize GSE reform in addition to headline risk," Kerins wrote.