"adamantly for" is a correct and usable phrase in written English.
It means to strongly support or stand firm in one's belief or opinion. It can be used in any situation where someone is expressing strong support or advocacy for something. Example: "The group of activists was adamantly for stricter gun control laws and held a protest outside of the Capitol building."
Once the institutions of British politics proved they could not hold — both major parties had pushed adamantly for the Remain cause — individual politicians hung out their banners.
They were adamantly for law enforcement and veterans' rights, in a manner that presupposed that the rest of us were adamantly against these things.
Until it recently backed down, it had cited environmental concerns for refusing adamantly for five years to meet the International Ski Federation's request to raise the starting-point of the men's downhill race to a higher altitude, in order to create a longer and more testing course than the hopelessly short one originally planned.
They bang the anti-abortion drum but are adamantly for the death penalty.
Do we know what our spending will achieve?" Even senators who were adamantly for tougher action were not happy with the measure they backed.
Jay rockefeller said in his statement that everybody uses to say that he's against the public option that he is adamantly for the public option.
And in the last few days, after adamantly insisting for so long that $1.6 trillion was "the right number" for a tax cut over the next 10 years, he has begun to concede that he will have to settle for a smaller figure.
The topic of gun control is extremely polarized; some adamantly advocating for their Second Amendment rights, while other concerned citizens plea for stricter laws and regulation.
She adamantly refused, for instance, to call him "Maestro".
But the Colombian officials who provided the computer files adamantly vouched for them.
That so many of Jun Takahashi's clothes for his Undercover label attest to the sensibility of a born designer but are adamantly geared for the street is proof that Mr. Takahashi has struck an essential balance.
