...As far as I know Obamacare has been mostly working ok, but sure let's rewrite the history.
Keep in mind that the ACA was originally Nixon's plan, then it was advocated for by the Heritage Foundation, and then it was the alternative to Clinton's plan proposed by Gingrich and Dole. The "individual mandate" is a conservative position, not a liberal one.
The major problem with the individual mandate is that it forces people to go to the private market without somehow instituting any form of price controls, whether through a regulatory body or by having a competing public option. As a result, private insurers, knowing people have to buy from them, have been able to hike their prices and make a killing from all the healthy twentysomethings who wouldn't have otherwise bought insurance.
That said, if the ACA had included a public option (IIRC it did in its original form but Obama wouldn't use the bully pulpit/threats of primary challenges to get it past the conservatives in his own party) the Republicans would want to kill it in order to "shrink government."
And the real reason for their objection to the ACA is the tax it imposes on people making more than $3,700,000 per year to fund the subsidies. The Republicans are the party of tax cuts, as long as you're really, really rich, and that's a tax they want cut.