I would think they are capable of doing this, but there is NO WAY they actually would.
Can you imagine the level of outrage that would result if it was ever proven that they would withold the news of Osama's capture until closer to the election? The sh*t would hit the fan in a major way.
Then again, I'm already imagining the reasons conservatives would come up with to defend such actions "Well, we were on the trail of other terrorists, and to release news of Osamas capture would risks us losing certain leads...it makes perfect sense. Only liberals would insist on doing otherwise"...
Bottome line is... no way is that true. Just too damn risky.