In Galatians Paul says that God’s promises were made to Abraham and his singular offspring (Christ), rather than to many offspring (the Jews):
Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” as of many. But it says, “And to your offspring,” that is, to one person, who is Christ. (Gal 3:16)
On top of that, Paul implies that Gentiles (more than Jews) are Abraham’s true descendants, the rightful heirs of salvation (Gal 3:6-9). But most importantly, God’s promises were made to a singular offspring: Christ. Bruce Malina and John Pilch comment:
“One might note here that Christian Zionists who support the Israeli state because they consider it the fulfillment of God’s promises to Abraham and his seed publicly and officiously deny what Paul says here. One wonders how they understand the basis for their Christian allegiances.” (Social Science Commentary on the Letters of Paul, p 206)
Romans presents a less radical argument, but certainly not enough to help the Zionist cause:
The promise [is] guaranteed to all Abraham’s descendants, not only to the adherents of the law but also to those who share the faith of Abraham. (Rom 4:16)
Now God’s promises are made not to a singular offspring (Christ), but to Jews and Gentiles equally. In Galatians Christ is the seed, and Gentiles favored. In Romans Jews and Gentiles are the seed, equally favored. (See my Romans commentary for an account of differences between the two letters.) In either case, one must wonder, along with Malina and Pilch, what bible Christian Zionists have been reading.