Republicans have been campaigning against Obamacare every since it was passed into law. Millions of Americans have access to healthcare today because Obamacare was passed over the objections of Republicans. However, that does not matter to Republicans. They repealed Obamacare over 60 times while Obama was President. That was easy because they knew that Obama would veto the repeals. They did not have to come up with a replacement for the bill that enabled millions of Americans to be covered by insurance.
The healthcare game changed when The Donald became president. They. can repeal Obamacare but they have to replace it with an alternative plan. They made one effort to replace it with another plan but they could not get enough Republican votes to pass the new plan. The Donald hates losing at anything. He, along with Paul Ryan, are getting ready to introduce a new plan so that he can declare victory. Paul Krugman provides an interesting way to describe the problem that they face. Imagine a balloon that is too big for box. If you try to push into the box by pushing on one part of the balloon another part of the balloon will inflate and keep you from getting it into the box. Now imagine that the size of the box is determined by how much money the government provides to subsidize the cost of insurance, and the amount of air in the balloon reflects the cost of benefits. The Republicans wants to reduce government funding which shrinks the size of the box. They can only do this by letting air out of the balloon. That can't be done without causing millions of Americans to lose coverage.
Krugman did not get too far into the details of Republican plan in his analogy. The editorial staff of the NYT describes some of the details of the proposals that the Republicans are floating so that Trump can declare victory. The net result of any plan to reduce the size of the box will cause millions of low income Americans to lose access to healthcare. Many of the most needy will not survive without health insurance.
Another reason why Republicans are so intent about repealing Obamacare is that they want to pass a tax reform bill that will lower taxes primarily for wealthy Americans and for Corporations. They hope to pay for the tax cuts by cutting government funding for Obamacare. Otherwise, the tax cuts will increase federal budget deficits. Trump's promise to cut taxes depends upon keeping his promise to replace Obamacare.