Taking over the offices in one or both of the two main parties has often been suggested as the way to get the right kinds of candidates, but it doesn't work. It could if the parties used the caucus system in each state and had a rule that there always has to be a "none of the above" (NOTA) option (as the Libertarians have). The trouble is that in most states there are primaries, provided for by statute, and a filing requirement that allows anyone to file, and if he spends enough money, win the nomination, even though he does not represent the positions of the party officials. It is possible for party leaders to influence the voting of caucusgoers, but not the general public who votes in primaries. That would require them to spend as much money as the candidate does, and he is probably backed by some special interests.
In other words, the two major parties have no firewall. There is nothing to prevent corrupt outsiders from seizing nominations. You can hold every office in the party and not be able to prevent that.
The primary system was sold as a way to avoid the undue influence of the "smoke-filled room" of party bosses. It did that, but it also opened the parties to the control of moneyed interests in a different way.