This brief describes a generalized method to realize an in-phase power splitter based on a four-port common-mode network (CMN). By enforcing one of the four ports open-or short-circuited, a three-port network with in-phase property and equal power division at two output ports is presented. A specific CMN consisting of two identical coupled-line sections is discussed and applied to the design of a wideband power divider with bandpass frequency response. The converted circuit can not only achieve the function of equal power splitting, but also flexibly allows for the control of equal-ripple bandwidth and in-band return loss (RL) level. Furthermore, the reflection coefficients and isolation of two output ports could be ideally enhanced by installing the synthesized lumped resistors. To confirm the design concept, a fully packaged wideband power divider working at 2.0 GHz with the fractional bandwidth (FBW) of 105% has been designed and fabricated using multilayer liquid crystal polymer (LCP) circuit technology. Final measured results, well agreeing with the simulated ones, exhibit wideband bandpass response, high port-to-port isolation, and port matching condition, as well as good in-band magnitude and phase difference.