We look for fluctuations expected for the QCD critical point using an intermittency analysis in the transverse momentum phase space of protons produced around midrapidity in the 12.5% most central C+C, Si+Si and Pb+Pb collisions at the maximum SPS energy of 158$A$~GeV. We find evidence of power-law fluctuations for the Si+Si data. The fitted power-law exponent $phi_{2} = 0.96^{+0.38}_{-0.25}text{ (stat.)}$ $pm 0.16text{ (syst.)}$ is consistent with the va-lue expected for critical fluctuations. Power-law fluctuations had previously also been observed in low-mass $pi^+ pi^-$ pairs in the same Si+Si collisions.
