WASHINGTON (AP) _ Calling Monica Lewinsky a ``very credible'' witness, a House prosecutor told senators in President Clinton's impeachment trial today that the former White House intern's testimony establishes that Clinton ``knowingly, intentionally'' sought to obstruct justice.

``If you believe the testimony of Monica Lewinsky, you cannot believe the president or accept the argument of his lawyers,'' Rep. Bill McCollum, R-Fla., said in a summary of evidence on behalf of the 13 House prosecutors on the second day of their presentation.

McCollum encouraged the senators to view the case against Clinton as ``one big obstruction'' in which the president ``thought his scheme out well'' in an effort to conceal his affair with Ms. Lewinsky from the court overseeing the Paula Jones sexual harassment lawsuit.

``If you believe the sworn testimony of Monica Lewinsky, if you believe her testimony that's in the record _ and she's very credible _ the president knowingly, intentionally and willfully set out on a course of conduct in December 1997 to lie to the Jones court, to hide his relationship and to encourage others to lie and hide evidence,'' McCollum said.