When I first started researching this costume to make it for my husband (over two years ago now!) I just assumed that the Senate Guards wore an inner cloak like the Royal Guards did.
This behind the scenes image shows a Royal Guard (on the right) without his red velvet robe, you can clearly see the burgundy colored robe with sleeves:
However, one time while watching "Star Wars: The Phantom Menace" I noticed a scene where the Senate Guards were on Naboo with Chancellor Palpatine. The wind was blowing the outer robes while the Guards were walking and it was very clear to me that they were wearing some sort of blue pants! I couldn't find any picture evidence of this, and I lack the programs that would allow me to do screen captures from the DVD, so I put the movie in my DVD player and took a digital picture. The images are pretty rough, but you can see what I saw: the Senate guards do not have an inner robe, they wear pants! It appears that the "shirt" and "pants" are all one piece like a coverall. The coverall does not appear to have a waist, there is no kind of belt, and no pockets. I did notice that one Senate Guard is wearing his pants tucked into the top of his boots while another is wearing the pants untucked.
(Click on an image to see it larger)
In this image, the Senate Guard on the right of the image is taking rather large strides. The wind is blowing his robe away from his legs. Pants can clearly be seen (untucked):
The Senate Guard on the left just behind Palpatine is another good example of clear evidence that he is wearing pants. If you watch the movie, the robe blows enough to show his waist...I believe there is no waistband, which leads me to believe this is a one-piece coverall sort of thing. His pantlegs are tucked into his boots:
The other Senate Guards in the background are also good evidence that they are wearing pants (tucked or untucked).
Another noteworthy thing that I noticed in several pictures, and should be replicated by anyone making/wearing this costume is that the sleeves of the under-"coverall" are worn OVER the gloves. (This can also be seen in the screen captures of mine above.)
I learned something new that was pointed out to me. Senate Guards also used Force Pikes! See this image from "Star Wars: Attack of the Clones"