Given that so many have said that terror warnings have been used for political purposes, is it possible that the current warnings about heightened threats are also politically motivated?
Well, Congress is currently voting on whether or not to renew the Patriot Act.
There is a lot of opposition to renewing the Patriot Act (and see this) and so - if people were going to use terrorism fearmongering for a political purpose - this would be a logical time to use it.
Indeed, fearmongering has been connected with Patriot Act extensions before.
Specifically, in 2006 - only hours after sensors in a U.S. Senate office building detected a nerve agent - key Senators suddenly reversed direction and announced a capitulation to the White House's demands on the renewal and expansion of the Patriot Act.
And when Congress was originally asked to pass the Patriot Act in late 2001, the anthrax attacks which occurred only weeks earlier were falsely blamed on spooky Arabs as a way to scare Congress members into approving the bill. Specifically:
The FBI was actually told to blame Anthrax scare on Al Qaeda by White House officials
High-level government insiders pointed towards Iraq as the source of the anthrax, even though there was absolutely no reason to think that the anthrax had come from Iraq