Despite contrasts in their political ideology, Democrats and Republicans have one thing in common this convention season: They're both preaching to the choir. 

According to Nielsen, the left-leaning MSNBC (Nasdaq: CMCSA) was the clear ratings winner among cable news networks that covered the opening of the Democratic National Convention on Tuesday. That's a decidedly different story from last week, when Fox News Channel (Nasdaq: NWS) led a clear victory over both MSNBC and CNN (NYSE: TWX) during coverage of the Republican National Convention.

MSNBC's Tuesday broadcast of the DNC, which aired from 8 p.m. to 11 p.m., attracted 3.3 million viewers. CNN pulled in 3 million viewers, proving that centrists may lean liberal after all.

Fox News, which typically beats both networks on any given night, attracted just 2.5 million viewers on Tuesday. However, the right-of-center network scored the biggest convention numbers yet during last week's RNC, topping off at 7.2 million viewers on Tuesday, Aug. 28. On that same night, CNN pulled in a paltry 1.5 million viewers.  

The most-watched hour during Tuesday's DNC broadcast was 10 p.m. to 11 p.m. when San Antonio Mayor Julian Castro and first lady Michelle Obama gave speeches. During that hour, all three cable networks were eclipsed by NBC, which attracted 5 million viewers. MSNBC came in second, followed by CNN, CBS and ABC. Fox News finished dead last.      

So far, Dems have overtaken Republicans in combined cable and broadcast viewers. During the peak 10 p.m. hour, the DNC attracted 21.9 million viewers compared the RNC's 20.5 million.

The ratings of both conventions agree with a wealth of media studies showing how consumers are attracted to news sources that align with their political viewpoints.