sentences:  When McClellan still failed to launch his attack, members of Congress urged Lincoln to replace McClellan with McDowell or Frémont, but Lincoln decided to retain McClellan as commander of Army of the Potomac over either potential replacement.He did, however, remove McClellan as general-in-chief of the army in May, leaving the office vacant.McClellan moved against Confederate forces in March, and the Army of Potomac fought the bloody-but-inconclusive Battle of Seven Pines at the end of May.