Obama is correct when he asserts that mandatory voting would be "transformative". The intellectually impotent mainstream right has no answer for why, in a country where democracy is venerated as the unquestioned (and unquestionable) apogee of political systems, anything other than universal suffrage should be the aspiration. March of progress, being on the right--er, correct!--side of history, etc.
As someone who is emphatically inegalitarian, it seems to me that electoral influence should in some way be associated with financial contribution to the apparatus of the state. Say, one vote for every digit of federal income taxes paid, starting with the third in the calendar year prior to the vote in question and net tax eaters being ineligible to vote at all. That is, someone who pays $200 in taxes gets one vote; $2000 gets two votes; $20,000 three votes, and so forth, with a maximum of say five or so votes.
I see no a priori reason why someone who contributes nothing to the treasury should have a part in determining how said treasury resources are spent, and certainly not a part on par with those who do contribute. If any company, organization, or even family (want your kids to do the family budgeting?) operated in that way, we'd be right to predict fiscal catastrophe to likely follow as a consequence. And so here we are.
Obama's party has obvious political interests in getting electoral activity from marginalized populations that currently can't (high schoolers, inmates, immigrants) or won't ('low information' citizens) vote, but bringing those segments of the population to the ballot box will just accentuate the problems the government faces. There is no conceivable way that we ever enjoy a stable currency or a balanced federal budget given the current system.