Recently, the use of a virtual backbone (connected dominating set) in various applications in mobile ad hoc networks (MANETs) has become popular. These applications include topology management, point and area coverage, and routing protocol design. In a mobile environment such as a MANET, one challenging issue in constructing a virtual backbone is to accomplish a distributed and localized solution that aims at balancing several contradicting objectives: small approximation ratio, fast convergence, and low computation cost. Many existing distributed and localized algorithms select a virtual backbone without resorting to global or geographical information. However, these algorithms incur a high computation cost in a dense network. In this paper, we propose a distributed solution based on reducing the density of the network using two mechanisms: clustering and adjustable transmission range. By using adjustable transmission range, we also achieve another objective, energy-ef.cient design, as a by-product. As an application, we show an ef.cient broadcasting where nodes (and only nodes) in a virtual backbone are used to forward the broadcast message. The virtual backbone is constructed using Wu and Li?s marking process [1] and the proposed density reduction process. The application of the density reduction process to other localized algorithms is also discussed. The ef.ciency of our approach is confirmed through both analytic and simulation study.