Less than six months ago, many political insiders were predicting Democrats would almost certainly take control of the U.S. House and even had the opportunity to take over the U.S. Senate.

Now, many of these same prognosticators are changing their tune as prospects improve for Republicans. The change didn’t happen overnight, but a clear trend has emerged since Republicans passed their signature piece of legislation, tax reform. And just as fortunes have improved for Republicans, history proves they can just as easily be reversed.

Since Republicans passed tax reform on December 20, 2017, all the key metrics for the party in power have vastly improved. According to political poll aggregator, RealClearPolitics, Trump’s average approval rating has increased a net nine points.

In the same period, the generic congressional ballot average, a measure of which party voters want to represent them in Congress, has moved from Democrats having a 13 point advantage to Democrats having just a three point advantage. This marks the highest point for Republicans and lowest point for Democrats for the 2018 election cycle.

And, voters are more optimistic about the direction of the country with 39{09f965da52dc6ab4c1643a77bd40d1f729d807040cd8db540234bb981a782222} saying the country is headed in the right direction, the highest level in nearly a decade and a net improvement of 13 points.

While the political landscape has moved more favorably towards Republicans, voter sentiment of the Republican passed tax reform legislation has improved as well.  When first passed, 30{09f965da52dc6ab4c1643a77bd40d1f729d807040cd8db540234bb981a782222} approved and 51{09f965da52dc6ab4c1643a77bd40d1f729d807040cd8db540234bb981a782222} disapproved of the legislation. Now, 37{09f965da52dc6ab4c1643a77bd40d1f729d807040cd8db540234bb981a782222} approve and 43{09f965da52dc6ab4c1643a77bd40d1f729d807040cd8db540234bb981a782222} disapprove, a net 15 point improvement.

To put tax reform’s approval in perspective, let’s compare to the Democrat’s signature piece of legislation, the Affordable Care Act (ACA), which was signed into law on March 23, 2010. On the day the Affordable Care Act was signed into law, 40{09f965da52dc6ab4c1643a77bd40d1f729d807040cd8db540234bb981a782222} of Americans approved and 50{09f965da52dc6ab4c1643a77bd40d1f729d807040cd8db540234bb981a782222} disapproved and the numbers remained static throughout the 2010 election.

In fact, it wasn’t until July 10, 2012, fully two years after the ACA was signed into law that it reached a net -6 approval that tax reform currently has. But the ebb and flow of ACA approval should serve as a cautionary sign for Republicans to not rest on their laurels. Although it took seven years, a plurality of voters now approve of the program.

Assessing voter approval of tax reform, a full 20{09f965da52dc6ab4c1643a77bd40d1f729d807040cd8db540234bb981a782222} have yet to make up their mind on the reform package. That leaves a big opportunity for both parties as they seek to define it. Millions will be spent in the upcoming election on whether the reform package is a tax cut for all Americans or a giveaway to the wealthy and corporate interests. Whoever prevails may be the one holding the keys to the House when the dust settles.