While there are definitely some similarities, it's worth noting that John Kerry is the son of a serviceman who earned a scholarship to Yale, fought and was honored in Vietnam, became a spokesman for veterans against the war and and called for accountability for U.S. war crimes, all before entering public office. His biggest flip-flop was admitting he was wrong about early support for the war in Iraq. He also paid his taxes.
Mitt Romney was the son of a millionaire who dodged the draft by serving a Mormon mission in a French palace for nearly three years, which he then claimed was basically like being homeless and struggling on the front lines. His biggest flip-flop is never holding a principled position in his life for any longer than was convenient, including running to the left of Ted Kennedy and promising gay Massachusans he'd be a champion for them, before shutting down shelters and suicide help-lines for gay teens and being surprised to learn gay people actually had families and kids. He never released his taxes in the campaign, because it turns out he was laundering money through his church to avoid paying taxes, in a manner now illegal.
I'm no McCain apologist, but it was hard to watch Kerry back-peddling from the sentiments he expressed as a spokesman for the veterans against the war during his campaign.
He found it too difficult to articulate his position as a patriot/soldier AND an anti-war protester to the American people, so he just distanced himself from his own reputation.
If he had remained the John Kerry that testified before the foreign relations committee in 1971 on the winter soldier investigation he might have won the election.
We are free to assume the worst about Romney's taxes because he obstinately did absolutely nothing to establish any reason to NOT assume the worst about Romney's taxes.