Republicans revolted over the line, calling it proof that Obama lacks belief in hard work and American entrepreneurs, and presumptive Republican nominee Mitt Romney has referred back to "you didn't build that" on the presidential campaign trail.
Yahoo News reported the cake as vanilla with vanilla frosting.
"Happy 50.99726th Mr. President," RNC spokesman Tim Miller wrote in an email. The decimal number tweaks the Democrats on an official blog post from White House Economic adviser Alan Krueger, who pointed out the July jobs report, released Friday, indicated the "unemployment rate ticked up to 8.3 percent in July (or, more precisely, the rate rose from 8.217 percent in June to 8.254 percent in July).
"23 million people struggling for work isn’t a rounding error," RNC Chairman Reince Priebus snapped back.
But the Democrats sent the cake back.
“This is typical of Mitt Romney’s approach to the middle class," DNC spokeswoman Melanie Roussell told The Hill. "He wants to ‘Let them eat cake!’ while robbing them blind. We sent the cake back to the RNC, along with a copy of the Tax Center’s report on Mitt Romney’s tax plan.”
The RNC also marked the occasion of Obama's birthday with faux e-birthday cards mocking gaffes by the administration.
Obama's campaign has used his 51st birthday as a fundraising opportunity, inviting donors to enter a raffle to attend his birthday bash Saturday in Chicago.
Updated at 5:42 p.m.
First photo provided by the RNC; second photo by the DNC.