"THE REPUBLICANS have to get in line. It is imperative that our own troops be disciplined." So says Edward Rollins, the White House political director, in what sounds almost like the opening salvo in a drive to purge Republicans who don't support Reagan administration programs.

The president himself sounds not at all eager to fight this kind of battle. "No, I don't agree," he said Wednesday. "And I intend to support as many Republican candidates as I can in the coming election year."

The president may remember what happened when Franklin Roosevelt set out to discipline his troops by opposing anti-New Deal Democrats in the 1938 primaries: all but one of the targets of that purge won. Since then, no president has openly opposed congressmen of his own party. They may have sensed that in a battle between a single congressman and a president, even a popular president, the voters have an instinctive sympathy for David over Goliath.

Actually what Mr. Rollins is proposing is party discipline, not a purge, and it is not so very obnoxious. Why should the president's party spend its money and its time supporting candidates who don't support the president's major policies or who insist on ridiculing him publicly? Every political party has its priorities, and it is hard to see why anti-Reagan Republicans have any moral entitlement to be top Republican Party priorities.

The problem is that it is hard to find any Republicans these days who are not, in some significant way, anti-Reagan. Mr. Rollins, in the same interview in which he calls for more party discipline, also calls for a compromise on the budget "in the not-too-distant future," and says that the administration's position on the Voting Rights Act makes it a "symbol of racism" to many voters. These are not exactly the president's positions as we understand them.

On those issues, Mr. Rollins' views are widely shared in Republican circles. The president's budget has so few votes in Congress that the Democrats' strongest bargaining threat is to bring it forward for a vote. There was no concerted effort among House Republicans to cure what the administration now says is the major defect of the House- passed Voting Rights Act.

The administration's problem is not so much disloyalty on the part of panicky Republicans--though there is some of that--as it is its own failure this year to craft policies on major issues that can win the support of the great mass of Republicans and some Democrats besides. This is the first political responsibility of any administration, and only after it has been met does it make much sense to talk of purges or party discipline.