Is the individual mandate payment penalty a tax?

As the dust settles after the Supreme Court speaks, the debate regarding this key element of the decision continues to claim the spotlight. 

The New York Times' The Caucus: Obama Aide Rejects 'Tax Label' For Health Care Law
Jacob J. Lew, the White House chief of staff, on Sunday defended President Obama's health care law but rejected the "tax" label that the Supreme Court, in a ruling that mostly upheld the law last week, used to describe the penalty payment required for those who choose not to buy health insurance (Lowrey, 7/1).

The Wall Street Journal's Washington Wire: McConnell, White House Debate Whether Health Law Is Tax
The Obama administration and Republicans battled on the Sunday morning talk shows over whether the health-care law largely upheld by the Supreme Court last week amounts to a tax. Senate Minority Leader Mitch McConnell (R., Ky.) attacked the new law, saying it amounts to a "middle class tax increase" that's largely going to affect Americans making less than $120,000 a year (Solomon, 7/1).

USA Today: Health Care Penalty Becomes A Sticking Point For Republicans
Republican leaders said Sunday that last week's Supreme Court decision upholding President Obama's health care law will backfire on Obama and Democrats, in large part due to Chief Justice John Roberts' ruling that a penalty on Americans who did not buy health insurance was legal because it was a tax (Kennedy, 7/1).

Bloomberg: Lew Says Republicans Should 'Get Over' Health Law Debate
White House Chief of Staff Jack Lew said Republican attacks on President Barack Obama's health care law as a tax are rehashing old arguments, as Republican leaders in Congress vow to keep up a drive to repeal it (Przybyla and Rastello, 7/2).