Politico has an interesting, deeply reported piece gaming out the possibility that Dems may end up holding on to the Senate, after all. It is, of course, always possible that Republicans will win it in the end. But in a cycle that initially seemed stacked in the GOP’s favor, Republican hopes have faded. Politico says this is a reflection of a fundamental problem in today’s Republican Party:

The GOP’s Senate challenges more broadly are a reflection of a party that simply cannot be controlled from Washington down — but also can’t reliably produce good candidates and winning strategies from the grass roots up.

The Senate takeover struggle of 2012 has revealed a central leadership that is unwilling, and perhaps unable, to control its base — enfeebled by fear of tea party activists, conservative talk show hosts and big-money outsiders who can swing primary several of the races — like Arizona, Wisconsin and Missouri — the party’s hands-off approach to primaries produced battered, weakened candidates who are struggling to pull away in races many Republicans thought were sure bets.

The Politico piece details a struggle between the GOP establishment and Tea Party base that remains unresolved, weakening the party’s prospects for Senate control. And this brings up an under-appreciated dynamic of the cycle: The surprising unity we’ve seen on the Democratic side between the base and what can be broadly called the Democratic establishment.

If you think about some of the leading Dem candidates — Elizabeth Warren in Massachusetts, Tammy Baldwin in Wisconsin, Martin Heinrich in New Mexico, and Sherrod Brown in Ohio — they are not just considered acceptable to the base and to the establishment. Both see them as exceptional, outstanding candidates, for political and substantive reasons alike. These candidates are considered progressive heroes by the base and its institutions, and they are considered excellent general election candidates by party leaders in Washington.

Meanwhile, the more moderate candidates — such as Heidi Heitkamp in North Dakota and Richard Carmona in Arizona — are also liked by the Dem base. Liberals like Heidi Heitkamp because she’s aggressively defending Obamacare. They like Carmona partly because he has an impressive set of qualifications as a former Surgeon General and Vietnam vet, and because he’ll help make the party more attractive to Latinos long term. The result is that the leading liberal institutions — labor, the League of Conservation Voters, Emily’s List, the Progressive Change Campaign Committee, the liberal blogoshpere — are playing heavily in the Senate races in a way that puts them, to a surprising degree, on the same page with Beltway Dems.

It wasn’t always thus. Labor and the left fought a bitter, divisive war with the Dem party committees in Arkansas in 2010, pitting their candidate Bill Halter against incumbent Blanche Lincoln. And back in previous cycles, as Politico details, the Dem party leadership pushed its chosen candidates past the base with a much heavier hand. But this time around, there’s relative peace and unity.

Dems still could very well lose the Senate if a few races break against them and if Romney wins the presidency and pulls along GOP candidates with him. But if Dems do hold the Upper Chamber, this surprising degree of unity may be a key reason why. And by the way, this also bodes well for liberals: If Dems hang on to it, the next Senate will likely gain a handful of new, high profile progressive warriors, who will pull the institution in a more liberal direction.