Since 1974 why does GDP rise with a new Labour government and fall with a new Tory government?

Well, if it's anything like our two main parties here in the U.S., one of the two is simply better with beneficial economics.

Like with the U.S., anytime you have a Republican-controlled Congress, the economy seems to do much better over the long-term. Any time you get Democrats controlling Congress, we have downturns rather rapidly that are blamed on the Republicans.

It seems to matter less who's president and more who's controlling Congress. The president sets policy, but no matter how disastrous he wants his policies to be, those who control Congress determine how that policy is implemented in a way that affect the economy.

So under both Clinton and Bush, our economy did better when Republicans controlled Congress, for the most part, and our economy started a decline only after Democrats took, control of Congress.

Both Bush and Clinton demonstrated this. Clinton came in on Reagan's wave of success with a Republican and Democrat Congress. Clinton had a Republican Congress to start, then got a Democrat Congress, and by the end of his term, we were on a decline again.

Bush came in, started doing polices with a Republican controlled Congress, and those policies were keeping us afloat regardless of the war spending, and then Democrats took over Congress, and shortly thereafter we hit another decline that led into the Great Recession of 2008.

That decline continued till a year after Republicans took the House. Note what happened a year after Republicans took back the House. We had that huge budget fiasco.

That was the very first time that Republicans forced Democrats to include them in economic legislation. Before that, Democrats had ignored all Republican input. However, after Republicans successfully forced Democrats to include them in economic legislation, our economy saw it's very first actual improvement after the Great Recession.

And we have only continued to do better the more Republicans have continued to have control.

Granted our GDP has slowed to a crawl, and our debt is now exceeding our GDP, and if we don't do something soon, we will be in a spiral of decline that we won't be able to escape, but hopefully, a Republican Congress will get something done.