Due to Political Intrigue, on December 18th, 1862, Grant split his forces
"If there is ever a fight here, it will be hard on us. Yet I have no fears about this place, I do not think the entire Federal army can take it." Capt J. Riddle, MS Artillery at Vicksburg
Just as today, political maneuvering in Washington D.C., impacts the people actually doing the job in the field. Former Illinois Congressman turned General, John McClernand convinced Lincoln to let him lead Illinois troops down the Mississippi River to capture Vicksburg.
General Grant concluded that by agreeing to McClernand's plan, Washington was interested in amphibious campaign down the Mississippi River. Lincoln, after experiencing slow moving commanders, was actually just trying to find someone willing to move forward. Bottom line - Grant decided he was going to start down the river first.
On December 18th, 1862, Grant split his forces. Admiral Porter is assigned to transport 30,000 of Sherman's men down to the mouth of the Yazoo river. Grant with the rest of the troops, will distract Vicksburg's defenders by marching southward, parallel to the Mississippi.
McClernand, by January 1863, became a constant source of friction for his colleagues during the Vicksburg Campaign. His constant attempts to remove Grant included spreading rumors to the press that Grant was drinking on the campaign.