PT "The dark side of the Force surrounds the Chancellor."

Palpatine didn't have as much power before the war broke out so the Jedi weren't as worried about him.

 

Yeah, before the war he was (outwardly) pretty much a nobody, he was pretty well thought of presumably but no-one would really have thought some respectable but not particularly notable senator from Naboo was going to rise like a rocket over the next few years to get Chancellorship. He just passed under the radar until the time the Dark Side clouded his real presence from detection. As per Darth Plagueis he actually managed to create the shroud of Darkness that gave him protection but that's presumably non-canon now.

 

The newcanon novel Tarkin has its own explanation for how the Jedi Order were "weakened" (as per Mace's "I think it's time we informed the Senate that our ability to use the Force has diminished"

"Alert me when the ship makes planetfall on Murkhana," Sidious said.
The droid bowed its head. "I will, Your Majesty."
The two of them were in Sidious's lair, a small rock-walled enclosure beneath the deepest of the Palace's several sub levels that had once been an ancient Sith shrine. That the Jedi had raised their Temple over the shrine had for a thousand years been one of the most closely guarded secrets of those Sith Lords who had perpetuated and implemented the revenge strategy of the Jedi Order's founders. Even the most powerful of Dark Side Adepts believed that shrines of the sort existed only on Sith worlds remote from Coruscant, and even the most powerful of the Jedi believed that the power inherent in the shrine had been neutralized and successfully capped. In truth, that power had seeped upward and outward since its entombment, infiltrating the hallways and rooms above, and weakening the Jedi Order much as the Sith Masters had secretly infiltrated the corridors of political power and toppled the Republic.
Save for Sidious, no sentient being in close to five thousand years had set foot in the shrine. The room's excavation and and restoration had been carried out by machines under the supervision of 11-4D.​

 

I agree with this. I also believe Mace was becoming so pessimistic by this point that he was more inclined than other Jedi to sense the dark side.

 

What makes you think that he was the only one who sensed it?

 

I think they all sensed it. After all, Obi-Wan voices his distrust of Palpatine to Anakin right from the start.

I think Obi-Wan has always been meant as a sort of neutral mouthpiece for the Jedi Order as a whole--kind of a halfway point between the self-reflective impotence of Yoda and the idealistic brashness of Mace Windu. Basically, Obi-Wan is the Super Mario of the Jedi--he embodies all of their best qualities as well as all of their flaws, but all in moderate and equal measure.

 

With the whole Vapaad-channels-Dark thing Mace had going on I guess there could be an argument he's more sensitive to it than most other Jedi - or perhaps more fearful/watchful for it than they are. Whilst the Jedi in general were suspicious of Palpatine it's not implausible he's more hardline in his mistrust; I seem to recall he's the one pushing for more direct action over Yoda's wait and see approach (which admittedly would not have worked any better long term as Palps/Sidious would have had too much political power to touch soon enough.)