Federal and postal unions are counting on Rep. Thomas S. Foley (D-Wash.), one of the newest and most junior members of the House Post Office-Civil Service Committee, to help block administration plans to cut federal salaries and extend pension requirements.

Proposals to cut pay 5 percent and make employes work until 65 to get full pension benefits are part of the budget President Reagan sent Congress Tuesday. None of the changes can be made unless approved by both the Republican-controlled Senate and the Democrat-dominated House.

Although next to the last in seniority on the civil service panel, Foley has considerable clout in Congress. He happens to be the No. 3 man in the House leadership (as Democratic Whip) and stands a good chance to be speaker next year. When a man in Foley's position says jump, most of his colleagues ask, "How high?"

Foley's decision to join the committee -- hardly a congressional prestige post -- surprised most of his staffers, and other committee members.

The committee acts as the port of entry, and often the roadblock, for most legislation introduced in the House relating to federal pay, fringe benefits or working conditions. Many new members assigned to it bolt for other committees as soon as they get enough seniority.

Its most famous graduate is former Rep. Geraldine Ferraro (D-N.Y.), who left it for other House assignments before being named Democratic vice presidental nominee.

During the first Reagan administration the committee's Democratic majority fought a largely successful holding action. It blocked passage of various White House-endorsed bills to change federal pay, retirement and insurance benefits.

Chairman William Ford (D-Mich.) is considered one of the most pro-labor members of Congress, and a staunch friend of federal-postal unions.

Insiders believe Foley sought the committee assignment in hopes of being named chairman of a subcommittee. That would give him an extra half-dozen staffers to help with House chores and with next year's campaign to succeed House Speaker Thomas P. O'Neill (D-Mass.) who is in his last term.

In the early organizing period of this Congress it seemed likely that several ranking committee members would move on to other assignments: Rep. Patricia Schroeder (D-Colo.) to Armed Services, and Rep. Mary Rose Oakar (D-Ohio) to Banking and Currency. But it now appears there are no vacant subcommittees on the civil service unit. This leaves Foley -- if he decides to stay on the roster -- as a very junior, but also very important, member of the committee.

"Foley has been our very best friend in the House leadership," a federal union lobbyist said yesterday. "We can count on him to stand up for us in leadership discussions and in House debates. A 'nod' from Foley is good for 50 to 100 votes.

"We wish he could have gotten what he wanted a subcommittee chairmanship, but we are delighted that he is on the committee."