You seem to have this habit of adopting definitions that suit your perspective.
"Fear mongering" doesn't imply that the danger isn't real. It may in Max-World, but that's not a faithful interpretation of the phrase.
I don't wish to reargue the Iraq War, but the fact that a fear didn't ultimately prove to be justified, doesn't automatically invalidate the fear when it was felt.
The whole world, including just about all of your left-wing heroes, believed that Saddam was pursuing, if not stock-piling WMDs. As it turned out he wasn't, but this doesn't mean the original fear was not reasonable.
The Bush Administration didn't assert Irag posed some WMD danger while knowing this was entirely false, but still they were guilty of "fear mongering."
There were several valid reasons for invading Iraq, but the Bush Administration, apparently, didn't believe the American people could comprehend them and so focused on fear of an Iraqi WMD finding its way to America.
That actually is "fear mongering."
Debt default is, of course, a real danger, but failing to raise the Debt Ceiling will not automatically result in America defaulting on its obligations.
If a deal cannot be reached, America will not default on its debts, unless Obama chooses not to pay our creditors.
There is sufficent revenue to not only pay the interest on our debts but to account for the financial obligations of Social Security, Medicare/Medicaid and the compensation and benefits of our military service men and women.
Something will have to give, but it won't have to be social security checks.
In this sense, Obama is guilty of even your definition of the phrase.
BTW - I hate to break it to you, but I don't believe Reagan was an infallable deity. That he said something doesn't, in my mind, make it so. All presidents are politicians and they all act accordingly. Some are worse than others, and Reagan was less worse than most.