The first Senate Democrat to support Defense Secretary-designate John Tower has frequently sided with Republicans but also cast crucial votes against three of former President Reagan's nominees.

Sen. Howell Heflin of Alabama was Reagan's most consistent Democratic supporter in the Senate, but he played a major role in the Senate rejection of Supreme Court nominee Robert H. Bork.He also cast the deciding vote in the Senate Judiciary Committee's rejection of two other Reagan nominees, William Bradford Reynolds for the number three post in the Justice Department and Jefferson B. Sessions for a federal judgeship in south Alabama.

Heflin met privately with President Bush for 45 minutes last week to discuss the Tower nomination. He said afterward that he believes a president's appointments should be confirmed unless it can be proven "that the person is flawed or that he would be a detriment to the country."

Had Tower been nominated for a lifetime judgeship, Heflin might well have voted against confirmation. But he said the since the secretary of defense serves at the pleasure of the president, Bush will be responsible and accountable for Tower's conduct.

Heflin, a 67-year-old ex-Marine and former Alabama chief justice, was widely criticized in Alabama for his opposition to both Bork and Sessions, but neither vote appears to have caused him any lasting political damage.

First elected to the Senate in 1978 with more than 90 percent of the vote, Heflin was re-elected in 1984 with 63 percent of the vote against a conservative GOP opponent.

His only announced challenger in 1990 is Republican state Sen. Bill Cabiness of Birmingham, a businessman who chaired the state's delegation to the Republican convention that nominated Bush for president.