WASHINGTON (Reuters) - A senior U.S. Treasury official on Thursday conceded the department must work to tamp down Congressional opposition to a financial backstop for mortgage finance giants Fannie Mae (FNM.N: Quote, Profile, Research) and Freddie Mac (FRE.N: Quote, Profile, Research) but said it was vital to get the proposals passed.

Treasury's acting under secretary for financial markets, Anthony Ryan, said repeatedly during a CNBC television interview he didn't think the troubled government-sponsored enterprises would need to tap unlimited credit lines but said it would help stabilize financial markets if they had it.

On Sunday night, Treasury Secretary Henry Paulson offered unlimited increases in lines of credit for Fannie Mae and Freddie Mac and said Treasury would buy stock in them, if necessary, to add capital.

The Federal Reserve said the shareholder-owned companies, which together own or guarantee nearly half of outstanding U.S. mortgages, would be able to borrow at its discount window if necessary until the Treasury plan was in place.

But lawmakers, especially Republicans, have expressed strong opposition to what some claim amounts granting a "blank check" to Paulson to help the GSEs. Paulson was on Capitol Hill on Wednesday trying to shore up support for the proposals.

"Obviously we need to work with Congress," Ryan said. "They understand the importance and role of stability in capital markets for our economy and we're optimistic that we're going to continue to move forward."

He noted Fannie Mae and Freddie Mac stock was widely held around the world, adding:

"The stability of these institutions is very important and having this ultimate backstop -- again, we don't see that they'll have to access this -- sends a very strong message in terms of the stability of the marketplace."

Asked whether specifying a dollar amount would ruin the effectiveness of the plan, Ryan said "I think it limits it, and it creates additional challenges for the marketplace. It looks different than it does when it's unconditioned and unlimited."