That is what Petraeus said during the conformation hearing:

The next U.S. commander for Iraq told Congress yesterday that pending Senate resolutions against a surge of more than 21,000 troops give the enemy hope by depicting America as divided on winning the war.
Army Lt. Gen. David Petraeus, President Bush’s choice to take over command in Iraq and change the tide of a deteriorating battle, revealed his assessment in questioning from the Senate’s two leading war hawks: Sens. John McCain, Arizona Republican, and Joe Lieberman, Connecticut independent.
Mr. McCain, who supports a larger troop increase, asked what the results would be if senators “tell those troops that we support you, but we are convinced that you cannot accomplish your mission. … What effect does that have on the morale?”
“Well, it would not be a beneficial effect, sir,” Gen. Petraeus replied during a confirmation hearing to four-star rank before the Senate Armed Services Committee. “Obviously, a commander would like to go forward with as much flexibility as he can achieve.”
Mr. Lieberman tried to pull Gen. Petraeus further into the upcoming Senate debate. He asked what effect an anti-surge Senate resolution would have on the enemy. After praising the U.S. system of open political debate, the general added, “having said that, a commander in such an endeavor would obviously like the enemy to feel that there’s no hope.”
“A Senate-passed resolution of disapproval for this new strategy in Iraq would give the enemy some encouragement,” Mr. Lieberman said, to which Gen. Petraeus answered, “That’s correct, sir.”

If Hagel and Bidden and the rest want to give voice to their displeasure at what Bush is doing, that’s fine but the consequences are what they are and that is giving hope to our enemy.
Petraeus also said that he needs the troops:

The new strategy for Iraq shifts the U.S. focus in Iraq to securing the population and creating secure conditions to enable the Iraqi government, Petraeus said. Solving security problems will not be the ultimate solution for Iraq, he acknowledged, but the Iraqi government cannot deal with the political issues it must resolve while violence in Baghdad creates life-or-death situations for citizens every day.
“The objective will be to achieve sufficient security to provide the space and time for the Iraqi government to come to grips with the tough decisions its members must make to enable Iraq to move forward,” Petraeus said. “In short, it is not just that there will be additional forces in Baghdad; it is what they will do and how they will do it that is important.”
When questioned directly, Petraeus said he would not be able to do his job as commander of MNFI without the additional 21,000 troops President Bush has pledged to Iraq. Deploying these additional forces will make it difficult to increase time between deployments for troops who have already been burdened by the war on terror, he said, but plans are under way to sustain increased force levels.

Who are Hagel, Warner and Biden to tell a General what he does or doesn’t need? Unless they want to put it all on the table and vote to defund, then they really have nothing to say about troop levels.