This is a big primary day—in fact, the last big test of where both parties stand before the November 2 election.

Candidates for US Senate, US House and gubernatorial nods will be decided in Delaware, Maryland, Massachusetts, New Hampshire, New York, Rhode Island and Wisconsin, while Washington, DC, Democrats will nominate a mayoral candidate (and it may not be incumbent Adrian Fenty.)

Any one result could be an outlier. But the patterns of results could tell us a lot about the status and character of the two parties going into the final run toward November.

Here’s why:

1. The GOP  and the Tea Party are Really Going at It.

The Republican US Senate primaries in Delaware and New Hampshire are clear tests of whether the GOP strategists in Washington or the Tea Party purists are going to define the future of the party. Parties leaders want state Attorney General Kelly Ayotte to be their Senate nominee in New Hampshire and Congressman Mike Castle to be their Senate nominee in Delaware. If Ayotte gets beat by conservative stalwart Ovide Lamontagne, who has the backing of the hyper-conservative Manchester Union Leader newspaper and the Tea Partisans, that will be a big deal. (And he will have done it without the backing of Sarah Palin, who surprised a lot of people by backing Ayotte.) If Christine O’Donnell, a genuinely wacky right-winger who is running with Palin’s backing, bests Castle in Delaware, the Grand Old Party really will be the Tea Party.

An additional test comes in Maryland, where a Palin-backed outsider, Brian Murphy, is mounting an aggressive challenge to former Governor Robert Ehrlich for the GOP nomination to challenge Democratic Governor Martin O’Malley. A Murphy win, which is unlikely but not beyond the realm of possibility, would reinforce the notion that Palin is the definitional national figure in the party.

2. Progressive Democrats Are Challenging Their Party’s Establishment As Well.

While much of the attention to Democratic Congressional primaries has been focused on Charlie Rangel’s bid for one more term representing Harlem (a contest that features a smart, idea-based challenge from union activist Jonathan Tasini), that race is really about questions of ethics and focus. In other states, there are some clear ideological tests for Democrats.

Three of the most important tests are in the New England.

In  Massachusetts, public-interest lawyer and union activist Mac D’Alessandro is mounting a serious challenge to disappointing incumbent Stephen Lynch in Massachusetts’ 9th District.

An open-seat contest in New Hampshire’s 2nd District features a clear choice between progressive Ann McLane Kuster and the much more conservative Katrina Swett, who is appropriately described as a "[Joe] Lieberman Democrat" by Progressive Change Campaign Committee co-founder Adam Green.

And in Rhode Island’s 1st District, progressive legislator David Segal has attracted strong progressive and labor support to a Democratic primary race for the seat left open by the retirement of Congressman Patrick Kennedy.  

3. Down-Ballot Progressives.

In lower-profile statewide races Tuesday, Democrats have a chance to nominate a number of fresh contenders with strong progressive credentials and endorsements.

Topping the list is New York State Senator Eric Schneiderman, who has attracted broad progressive support in his run for the Democratic nomination for attorney general, one of the most powerful state positions in the country.

Keep an eye, as well, on Joe Fernandez, a law-school classmate of Barack Obama, who is seeking the Democratic nomination for Rhode Island’s attorney general. And Suzanne Bump, who is backed by Progressive Democrats of Massachusetts, looks likely to win the Democratic nomination for state auditor.