President Reagan said Friday he supports a 50 percent pay raise for members of Congress and other federal officials because "we must not allow federal service to become the province only of the wealthy."

In a letter to the House and Senate, Reagan said he endorsed the pay-raise recommendations of the Commission on Executive, Legislative and Judicial Salaries. He said "fair compensation for those who bear the responsibility for effective functioning of our government is critical at this juncture of history."In advance of the president's decision, which was made public late Thursday by the White House, consumer activist Ralph Nader labeled it the "greatest salary grab in government history."

The raise will increase the pay for members of Congress and federal judges to $135,000 a year unless both houses of Congress reject it by Feb. 8.

In his letter Friday, the president coupled his support for the pay hike with a plea that Congress at the same time bar outside income such as speaking fees, as the commission recommended last month.

"The current system undermines public faith in the integrity of senior federal officials, particularly in the legislative branch," Reagan wrote. "Congress should move immediately to enact legislation that takes a comprehensive approach to the problems posed by honoraria, including payments for articles, speeches and appearances, and other forms of financial benefits."

President-elect George Bush, asked Friday about the pay raise, said he supports whatever Reagan decides but may take another look at the issue when he becomes president.

Asked by reporters if the nation could afford the big pay increase at a time when it is grappling with huge budget deficits, Bush said: "Well, I don't know whether we can afford anything other than to get this deficit under control. I am vice president and I will be supportive of what the president decides. I've done that for seven and eleven-twelfths years and I don't plan to change now. So let's see what the president says and then when the ball is in my court and the buck is stopping on my desk, I may have something else to say about matters of this nature," Bush said.

He spoke after the White House released Reagan's letter supporting the pay increase.

Rep. Vic Fazio, D-Calif., said, "I'm very pleased that the president has decided to make this decision. This is a pay raise for all the top-level federal officials . . . but, of course, we (Congress) take all the heat."

Nader said, however, "Mr. Reagan has put an end to any further pretense that he wants his federal deficit reduced."

The commission recommended that members of Congress and federal district judges, who now make $89,500 a year, be paid $135,000. The House speaker's salary would go from $115,000 to $175,000, and that of majority and minority leaders from $99,500 to $155,000.

Top executive-branch officials such as Cabinet members would get raises from their current $99,500 to $155,000.

The commission also recommended that Congress raise the president's pay to about $350,000 from the current $200,000, where it has stood since 1969.