Not to mention which court, if any, would hear the case if the president were to appeal. A subpoena, scholars say, might set up a dangerous and unprecedented constitutional standoff. And then again it might not. Precedent only takes you so far. Clinton's best chance to defeat a subpoena might be what legal scholars call the ''separation of powers'' argument.
History offers only limited guidance.
Presidents going back to Thomas Jefferson have complied with subpoenas issued by criminal and civil courts or congressional committees. In 1926, the Supreme Court held that a presidential appointee could be arrested by the Senate to force him to answer a summons. But the issue has never come up in an impeachment. Seven of the 14 federal officials ever tried by the Senate have testified for themselves. But all did so voluntarily.