The phrase "a major and decisive" is correct and usable in written English.
It can be used to emphasize the significance and impact of a particular action, decision, or event.
Example: "The committee's decision was a major and decisive factor in the project's success."
Alternatives: "a significant and determining" or "a crucial and conclusive".
So far this election season, the Tea Party has brought a huge amount of unexpected energy into the campaign, and it could drive sufficient Republican turnout to become a major and perhaps decisive factor in many races.
If so, that would be a major and perhaps decisive blow to content-involving computationalism.
With at least 200,000 boricuas having arrived in Florida since the last presidential election in 2012 – all of whom are eligible to vote at the age of 18 as US citizens – they have the potential to be a major, and possibly even decisive, political force in a state that gave Barack Obama the nod four years ago by only 74,000 votes.
The strategy, in my view, should be federal support in return for a major, historic, and decisive retooling to energy-efficient vehicles.
Air power played a major, perhaps decisive role in blunting Iraq's ability to resist.
The authors of only two of the ISAAC-type studies seriously questioned the causality of the detected association [ 28] or concluded a major or decisive role for confounding factors [ 32].
The key aspect of Sachs's recommendation is to be found in the words "major, historic, and decisive retooling," as a condition for aid.
If he makes an observation about, say, Gen. Robert E. Lee's failure to win a major decisive battle against the Union Army, he will follow it with sentences like these: "Indeed the era of decisive battles was drawing to a close.
Activation of the α-SMA-positive, matrix-producing myofibroblasts has long been recognized as a major decisive event in tissue fibrogenesis after chronic injury.
So to a fifth, and decisive set.
