The Third Party Dialogue: How Can Centrists Reach the Democrats?

Happy New Year!  As noted last week, the New York Times held a readers' dialogue on the issue of third party politics that began with a letter to the editor by Robert Levine calling for a centrist alternative to the Democratic and Republican parties.  Over the weekend they published nine responses to the original piece plus a response from Levine himself.  It may be interesting to simply layout the general tendency of each response, letter by letter, to see the breadth of opinion and overall tendency of the discussion.

1) Historical determinist argument against third party activism, i.e. third parties have failed in the past, therefore they will fail in the future.  Third party advocacy will further splinter the electorate.  It is better to work within the two-party system.
2) Disagreement with Levine's assumption that we need a third party of the center, states that Democrats and Republicans are already centrist-oriented.  Rather, we need a third and fourth party of the left and right to more adequately represent the breadth of opinion and interests among the people.  We need campaign finance reform and electoral reform to level the playing field.
3) Any third party strategy should focus on the US House not the presidency.  Reformers can also target competitive primary elections in the major parties to oust incumbents. 
4) Third party representatives would be just as corrupt as their Democratic and Republican counterparts, and would be sucked into the establishment's political machine.  Democrats and Republicans need to reform their respective parties and institute campaign finance reform.
5) "Centrism" got us into the current mess.  The problem isn't lack of centrists, it's obstructionism on the part of Republicans.  We need real opposition to the political and economic establishment in the form of a progressive third party.
6) A third party is not the answer and the barriers to entry are too high in any case.  Independents need to become involved in the major parties' primaries to moderate their extremes.
7) The United States already has numerous third parties but they are ignored by the media and marginalized by the political establishment.  We need to hear all positions across the political spectrum and not presented with a false choice between Republicans and Democrats.
8) The Democratic Party is already "the" centrist party.  Republican obstructionism is the problem.  History implies that third parties will not win and act as spoilers.
9) The Democrats are already "the" centrist party.  We need multiple viable alternatives to the major parties.  For that to happen we need serious electoral reform: campaign finance, voting systems etc.  

Read Levine's response.  One issue he does not touch on in his response to the response is the argument that the Democrats are already "the" centrist party.  This is a very common argument, at least in the mainstream liberal Democratic blogosphere, and directly contradicts the centrist position that the Democratic party has been captured by its far left progressive wing.  One obvious retort that one often hears in centrist circles is that the Democratic base is so far to the left that they think their party is centrist!  This argument leaves much to be desired as it relies on nothing more than the assertion of political relativism and ideological false consciousness.  Given that, politically speaking, Democrats are much less independently-minded than even Republicans, centrist third party advocates are going to need to develop arguments to appeal to those who would otherwise continue to cast their ballots against the Republicans by voting Democrat.  Ideas?