Public Campaign Action Fund's David Donnelly has a letter in today's Washington Post explaining the distinction we've been talking about when it comes to how Sen. John McCain (R-AZ) and Sen. Barack Obama (D-IL) measure up on the issue of public financing of elections: Obama has pledged support for public financing of federal elections, McCain has not.

From the letter, why choosing to opt in to the presidential public financing system is not the most important marker:

Each candidate will make a decision to opt into the presidential public financing system according to whether doing so will help him or her win.

The principle to which we all ought to look for guidance as to whether a candidate supports campaign finance reform is whether that candidate has pledged to make public financing of all federal races a priority if he or she is elected.

Mr. Obama has given that pledge. Mr. McCain hasn't.