  We study URu$_{2-x}$Fe$_x$Si$_2$, in which two types of staggered phases
compete at low temperature as the iron concentration $x$ is varied: the
nonmagnetic "hidden order" (HO) phase below the critical concentration $x_c$,
and unconventional antiferromagnetic (AF) phase above $x_c$. By using
polarization resolved Raman spectroscopy, we detect a collective mode of
pseudovector-like $A_{2g}$ symmetry whose energy continuously evolves with
increasing $x$; it monotonically decreases in the HO phase until it vanishes at
$x=x_c$, and then reappears with increasing energy in the AF phase. The mode's
evolution provides direct evidence for unified order parameter for both
nonmagnetic and magnetic phases arising from the orbital degrees-of-freedom of
the uranium-5$f$ electrons.
