The 77th Congress (1941-1943) has 66 Democrats and 28 Republicans in the senate with 2 independents; in the house, there 267 Democrats and 162 Republicans with 6 independents.
Whatever pressure the Republicans were exerting on FDR couldn't be that much.
There were many reasons for our policy towards Japanese-Americans - some of them good, many of them bad. This was just one of the many tough choices Roosevelt had to make.