Why would you think that?
First of all the premise doesn't even make sense. These people are hinting that police chiefs are resigning due to an impending martial law declaration. Martial Law means that these chiefs would be the last ones to know, since it effectively bypasses their departments.
In order to support this premise the advocates would have to show the following:
1) Police chiefs are resigning in larger numbers than 'usual'
2) That the reasons given for their resignations were false.
3) That the true reason for their resignations was "something to do with Obama"
4) That the "something to do with Obama" was something an impending declaration of martial law
5) That the Chiefs were aware of this.
By "Police Chief from Michigan" I'm assuming you mean Warren Evans, who resigned as the Police Chief of Detroit in late July.
"I got some information on some things that had happened in our police department," Bing said at news conference. "I thought Chief Evans, some of his decisions, I think compromised his position as chief, and because of that, I've made the decision to replace him."
Doesn't appear to have anything to do with Obama, and more to do with a lack of confidence in the Chief by the Mayor (i.e., his boss).