I think that TPM alone shows the Republic to be a deeply flawed institution:

“I can’t beleive there’s still slavery in the galaxy, the Republic’s anti-slavery laws…”
“The Republic doesn’t exist out here.”

“Enter the Bureaucrats, the true rulers of the Republic…and on the payroll of the Trade Federation.”

“The Courts take even longer to decide things than the Senate.”

“It is clear to me the Republic no longer functions.”

These problems definitely existed before Palpatine was Chancellor, and almost certainly before he was a Senator.