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The steady-state response of forced single degree-of-freedom weakly nonlinear oscillators under primary resonance conditions can exhibit saddle-node bifurcations, jump and hysteresis phenomena, if the amplitude of the excitation exceeds a certain value. This critical value of excitation amplitude or critical forcing amplitude plays an important role in determining the occurrence of saddle-node bifurcations in the frequency-response curve. This work develops an alternative method to determine the critical forcing amplitude for single degreeof- freedom nonlinear oscillators. Based on Lagrange multipliers approach, the proposed method considers the calculation of the critical forcing amplitude as an optimization problem with constraints that are imposed by the existence of locations of vertical tangency. In comparison with the Grobner basis method, the proposed approach is more straightforward and thus easy to apply for nding the critical forcing amplitude both analytically and numerically. Three examples are given to conrm the validity of the theoretical predictions. The rst two present the analytical form for the critical forcing amplitude and the third one is an example of a numerically computed solution.