God, I hope so.
Oklahoma has one of the highest rates of interracial marriage. Alaska too.
A possibly more workable approach might be to increase the tax credit for having kids, but also allow it to be taken in the form of reduction of student debt.
I’ll reserve other notions of how to encourage smart people to have kids for another thread and not further hijack the topic of this one.
While the diaphragm is more than a century old, generic-priced birth control pills are a lot newer. Even newer is the situation where only a few social groups, that tend to keep to themselves, are maintaining a higher than replacement birth rate.
It used to be that birth rate wasn’t as important a factor in population change as the chances of a person living long enough to parent. Living in a world where obesity is perhaps more of a problem than famine totally changes our demographic future, just as much as medical birth control.
Some children, from high birth rate groups, do drop their parent’s religion. If so many do it so that the long term fertility rate of the group goes below 2.1, that group will also go into decline. This might already be happening to the Mormons.
By the way, high birth rate does not necessarily translate to conservative. The Amish, and the religiously related Hutterites, are pacifist.
Of course the future is unpredictable, but here we are allowed to predict anyway 
I predict we are close to peak atheism.
I also think/predict that tax rates are much less of a factor in fertility than religion.
Ok, let me explain why there’s an obvious objection to such a system: the Constitution requires equal treatment of all people. The Supreme Court has held that this means that all marriages (between two consenting adults blah blah blah) must be treated equally by the government. See: Loving v. Virginia, Obergefell v. Hodges, etc… This means that your plan would be unconstitutional.
This proposal is a shortcut, or easy way out, that over time would prove to obtain the opposite of what’s intended. Never bring money into it. People will just say, “See? It’s reverse discrimination.” You want them to say, “Being racist is a bad idea.”
Thomas Jefferson, a guy who had a lot to do with building freedom into governmental processes, suggests in cases where ignorance seems to be a trigger of some sort, rather than legislate around it, have an education process which informs people to such a degree these ideas they may hold which are destructive by nature will become obviated as really bad ideas.
Tom was pretty smart, and his writings can be seen as an owner’s manual.
Always educate before you legislate. You might be surprised.
And don’t go bitching about the education system. It can be repaired…with the cost of one ICBM.
[Senator Jay Billington Bulworth]
“All we need is a voluntary, free-spirited, open-ended program of procreative racial deconstruction. Everybody just gotta keep fuckin’ everybody 'til they’re all the same color.”
[/Senator Jay Billington Bulworth]
Can’t believe that’s not been posted yet! 
I don’t think you can say that the Amish, Mennonites, or Moravians aren’t conservative just because they may be pacifists. Quakers are often pacifists. Nixon was a Quaker. And insisting that your members dress in a fashion that went out of style 150 years ago is fairly conservative.
A wealthy black man marries a wealthy white woman. They have mixed kids.
What problems does this solve?
Hi. I want to use you for your race, so we can get a break on our taxes. Buy you a drink?
Why not? It’s been known to work to get a contract that supposedly should only go to a female or minority business.
Racism isn’t an education problem. The education against racism has been available for a couple generations now. If you watch any mass media, you will learn that racism is wrong and why.
Now you can argue there’s an issue with not accepting what they are taught as true, but that isn’t resolved with more education.
Not that this legislative idea has any merit, for a simpler reason than others have stated: the proposal itself promotes racism, legislating it into the law. It is only switching around who you get to be racist toward. Now it’s anyone of insufficient ethnic/racial mixture. Anyone who happens to fall in love with someone who is of similar race to them is penalized for choosing to marry them. And marriage is a right.
It even has practical difficulties: if you are of mixed heritage, how do you find someone else who is sufficiently different? You have the perverse issue where people will want to pick as little diversity as possible to keep the incentives coming in.
Of course, there’s also the stuff others said about how even the goal doesn’t actually fix racism: unless everyone looks exactly the same, those who want to be racist will find a way to discriminate. Colorism is definitely one of those ways, but any physical attribute would count as racism.
And, yes, it is ultimately a form of eugenics: you are trying to use selective breeding to produce offspring with what you consider desirable characteristics. Thus the same issues with other forms of eugenics apply here. It’s not a scare term, just an accurate one. It reduces it to a solved case: eugenics is morally wrong. This plan is a form of eugenics. Therefore it is morally wrong.
Yeah - other than direct reference in the 1st sentence of the OP! 
There are already a lot of economic incentives toward intelligence-based assortative mating, and it’s not at all clear that it’s a good thing on net.
We should give tax breaks to encourage marriages between people who each have four health, active grandparents…
Life is short, but the years are long…
…Not while the evil days come not.
I think we already have enough unstable marriages when couples are supposedly entering into them for no reasons other than a desire to be married to each other. Incentivizing people to get married for a tax break would undoubtedly lead to an increase in unstable marriages and I feel the society as a whole would suffer from this increase.