Sixteen Republican senators sent a letter to President Reagan yesterday supporting Melvin E. Bradford, a longtime Reagan supporter and strongly conservative Republican, for the chairmanship of the National Endowment for the Humanities.

"Dr. Bradford's credentials as a scholar are impeccable," the letter states. "He is the author of over 130 essays as well as three books." The senators also note, "Dr. Bradford worked long and hard for your election in both 1976 and 1980. As a former conservative Democrat, he was a key figure in the Texas Republican effort to bring conservative Democrats into the Republican column."

The letter, which bears the heading of the Senate Republican Policy Committee, was characterized by one White House staffer familiar with the campaign for the NEH chairmanship as "a last-ditch effort on the part of Bradford supporters."

"Maybe it is," said Jerry Woodruff, an aide to Sen. John East (R-N.C.), one of Bradford's staunchest supporters and a signer of the letter. "The members who signed the letter definitely want the president to know how they feel." The Senate must confirm the NEH appointment.

William Bennett, director of the National Humanities Center in Durham, N.C., is thought to be the other finalist for the NEH post. However, the two North Carolina senators are supporting Bradford, a Texan and an English professor at the University of Dallas.

Other signatories of the letter of support include Jesse Helms (N.C.), John Tower(Texas), Strom Thurmond (S.C.), Orrin Hatch (Utah), Jeremiah Denton (Ala.), Dan Quayle (Ind.) and James McClure (Idaho).

The authors of the letter list members of the intellectual community who support Bradford: "Russell Kirk, Jeffrey Hart, Bill Buckley, Gerhart Neimeyer, M. Stanton Evans, Andrew Lytle, Harry Jaffa, and dozens of others. Dr. Bradford helped establish Scholars for Reagan and worked on the transition team for NEH."

The White House, which was expected to name a candidate for the NEH post at a White House luncheon last Wednesday, has apparently still not chosen one. Amid heavy campaigning, Bradford has emerged as the most controversial and the most high-profile aspirant. "One hundred fifty letters of endorsement for him have been written," said Charles Goolsby of the Senate Republican Conference.

Bradford's detractors are mainly East Coast neo-conservatives. Most prominent among them is Irving Kristol.

Braford has drawn much attention in the last month for having been a supporter of George Wallacein the 1972 Democratic presidential primary and for being a critic of Abraham Lincoln. "I don't criticize him for freeing the slaves," said Bradford, who has published on the subject.

Bradford criticizes Lincoln for his handling of the Civil War, his "misinterpretation" of the Constitution and for being morally "hypocritical." Said Bradford, "When he ran for the Senate, he said slavery was awful and terrible but he wasn't going to change it."