What is the difference between “Strike the match” and “Strike a match”?

Oishi-san, the match is part of an extended metaphor used by the writer. Looking further back in that article, we see

After a party-line vote by the court to decide the disputed 2000 election for George W. Bush over Al Gore, and another in the controversial Citizens United campaign-spending case, the Washington atmosphere reeked of gasoline, and the Obamacare case looked like a match ready to fall.

The image is of gasoline all around, rendering the President's agenda extremely combustible, and a match (Obama's health care bill) being readied to ignite it. Roberts' initial words seemed to be "striking the match" that would set fire to the gasoline.

So in this case there is no difference between the two expressions save use of the definite article to refer to a particular match being struck.