In case anyone didn’t know that Barack Obama’s liberal base didn’t like the deal he cut on tax rates, Gallup’s new poll shows more erosion in his standing on the Left.  Erosion in this case is a relative term, however, as his approval rating from self-described liberals in their survey dropped from hero worship to … mere adulation:

Liberal Democrats remain strong supporters of President Obama, but their approval of the job he is doing has fallen noticeably since the midterm elections. For the first time, it dropped below 80% in the week after the announcement of the tax deal he brokered with congressional Republicans. …

The Gallup data indicate that Obama’s support among liberal Democrats was starting to decline even before he reached the tax deal. He averaged 88% approval among this group the last full week before the midterm elections (Oct. 25-31) and 83% the first three full weeks (Nov. 8-28) after his party suffered major losses in those elections, and then dipped below 80% the week after the announcement of the tax deal on Monday, Dec. 6.

Democrats in Congress largely opposed the deal, mainly for extending the 2001 and 2003 income tax cuts for wealthy Americans and for its revisions to the estate tax. Those revisions included higher limits than Democrats wanted on the amount of inheritances exempt from taxes and lower tax rates on the portion of inheritances that are subject to taxes.

Liberal Democrats have been Obama’s most consistent supporters throughout his presidency, averaging 89% approval since he took office. That compares with an 82% average approval rating among moderate Democrats and a 75% average among conservative Democrats during his tenure in office.

To some extent, these will be the wages of triangulation.  In order to move to the center successfully, Obama will have to disappoint the Left, probably more than occasionally.  The tax deal that left the status quo on income-tax rates and watered down the estate tax hikes already scheduled were two key policy decisions that progressives wanted to defend.  They may decide that extending the federal UI programs and getting more subsidies for ethanol are enough of a trade off to avoid holding a grudge, but they won’t be happy about it and will lose some enthusiasm for the most progressive President in decades, if ever.

However, these numbers don’t amount to a collapse among the Left, or anything like it.  While progressive icons like Ralph Nader blast Obama as a “con man,” four out of five liberals still approve of Obama’s job performance.  There doesn’t seem to be a mass exodus on the Left from Obama’s banner.

That doesn’t mean this isn’t a problem for Obama.  This strategy will only succeed if Obama picks up support in the center that outpaces what he loses on the Left.  So far, at least, that’s not happening.  While Obama lost nine points among liberals, he also lost five points among conservative Democrats, two points among pure independents, and a point among moderate or liberal Republicans.  In fact, he only gained among one constituency: conservative Republicans, and that only a +2 from 6% to 8%.

Obama has about a year for triangulation before having to move rhetorically to the Left to shore up his base for the next election, so this is merely the start.  If he doesn’t start selling his centrism a little more effectively, that small erosion on the Left might be enough to make him a one-term President.