For generations, it has been fashionable among grumpy Republicans and triumphant Democrats to observe that Somerville is a one-party city.

While the tectonic shifts of local politics may still be largely under the surface, the rumblings of change are growing louder. It is increasingly obvious that for the first time in living memory, Somerville does indeed have two political parties: old-school Democrats, and Progressives.

Though nominally all Democrats, these two factions look for all the world like rival parties – they hold different ideals, are concerned with different priorities, and compete with each other for power, influence, and electoral positions.

The most recent incarnation of the Progressive Democrats in the city, born out of the ashes of Robert Reich’s failed gubernatorial campaign, have moved quickly and aggressively to stake their claim. Their organization and discipline allowed them to infiltrate the Democratic ward committees and elect their own candidates to ward offices.

The potency of the new progressive force is evident in the tenor of Somerville’s statehouse races this year. In the 26th Middlesex district, Avi Green and incumbent Timothy J. Toomey Jr. have vied in their debates and campaign literature for the title of “most progressive.” Both candidates sought the endorsement of the Progressive Democrats of Somerville.

In the 34th Middlesex district, incumbent Vincent P. Ciampa has also found himself challenged by a Progressive candidate, Carl Sciortino. Sciortino has attacked Ciampa on his opposition to gay marriage and abortion.

Like Toomey, Ciampa seems not to have seen this threat on his left flank coming. In fact, so far Ciampa seems to want to just wish the Progressives away. He challenged Sciortino’s nomination papers earlier this summer, clearly hoping that his opponent would blow away in the first strong wind.

What Toomey, Ciampa, and the Democratic old guard are discovering is that the Progressives have all the elements of staying power: organization, dedication, a growing demographic base, and a well-articulated platform.

It remains to be seen how well the old guard, so long accustomed to a monopoly on power, can adjust to a vigorous political challenge.

One thing is certain, however. The city that has always loved a good party now has two.