Why are Republicans acting this way?

First, some background:

-Prior to the recent debt ceiling fight, the debt ceiling had been raised 74 times since 1962 without controversy and without conditions. The vast majority of current Republican who were in Congress during the Bush administration voted for increases each of the 7 times it was increased. These Republicans now decided that trillions in cuts must be attached to the recent increase.

-7 Republican Senators (Robert Bennett of Utah, Sam Brownback of Kansas, Mike Crapo of Idaho, John Ensign of Nevada, Kay Bailey Hutchison of Texas, James Inhofe of Oklahoma, and John McCain of Arizona) co-sponsored a bill to set up a deficit commission which Congress would be forced to vote on. After Obama came out in support of the bill, all 7 removed their names from the bill and voted against it. As a result, we got a weaker commission without a binding requirement for a vote.

-During the health care reform debate, Republicans stated they wanted 4 things in the bill:
1. Allow people to buy insurance across state lines. This is in Obama's law.
2. Allow individuals and small businesses to pool together to buy insurance. This is what the health insurance exchanges established by Obama's law are.
3. Allow states flexibility to experiment with different approaches. This is in Obama's law, which allows states to opt out if they have a better proposal.
4. Tort reform. No particularly strong elements of this in the bill, though it hasn't particularly been shown to work when it comes to limiting health care costs.

Despite including these elements in the law, as well as support from such noted Socialists as Bob Dole and Obama's refusal to push for a public (government-run) option, Republicans uniformly voted against it and decry reform as an affront to all that is right and American.

Why are Republicans acting this way? Why do they demonize proposals which they used to support, just because Obama supports them? Why do they refuse to compromise with him, rejecting deals like Obama's "Grand Bargain" which included between $3 and $5 in spending cuts for every dollar of tax increases?

Republican Senate Minority Leader summarized the Republicans' position very succinctly when he said "the single most important thing we want to achieve is for President Obama to be a one-term President." He didn't say their number one goal is to put Americans back to work, or let them keep more money in their pockets, or anything else not hyper-partisan. Their goal is to make Obama fail, and simple game theory demonstrates that this can't happen by giving him anything he wants.

There are two possible scenarios come Election time; either the economy improves, or it doesn't. If it improves, history tells us that Obama would be more likely (though not a lock) to be re-elected, whether he had some Republican support or not. If it doesn't improve and Republicans had supported some of Obama's initiatives, Obama would be able to share the blame with them, much as Bush stuck John Kerry and the Democrats with some blame for Iraq during the '04 election. If it doesn't improve and Republicans voted against him all the way, he gets all the blame and is an easy target come 2012.

So, given that their stated #1 goal is to make him a one term President, it makes sense that they would deny everything to Obama. There is even motivation for them to actively prevent the economy from improving. Given their refusal to support previously non-controversial things like raising the debt ceiling and infrastructure spending, there's reason to think that's what they're doing.