Far be it for me to dispute the Conventional Wisdom that says last night's elections in Virginia and New Jersey "reveal[ed] cracks in [the] Obama coalition." Still, there are other possibilities based more on evidence than opinion.
Virginia governor - Virginia has a history of electing governors from the president's opposition party. Virginia was a stronghold for George W. Bush, yet elected two Democratic governors during his administration. No one thought either of those were a referendum on Bush.
New Jersey governor - New Jersey voters had a mad on for Jon Corzine well before Obama was elected. Corzine did little to curb their anger. This was more of a "and the horse you rode in on" election than a referendum on national politics.
One race that might be realistically viewed as showing a trend was the special election in New York's 23rd Congressional District. Three candidates ran: a Democrat, a Republican, and a Conservative. (There is an established Conservative Party in New York.) The Republican candidate was trashed for not being conservative enough; "opinion makers" such as Rush Limbaugh, Glenn Beck, Sarah Palin, and Michele Bachmann loudly endorsed the Conservative. The Republican candidate, turned on by her own party, dropped out and said the Democrat was a better choice than the Conservative. The Democrat won handily.
Conservatives went out of their way to make this a national election, and were trounced. The Republican party finds itself in a similar position to Muslims. They either need to speak out against their extreme branches, or risk becoming condemned with them.