Why would a Republican veto his own Party's spending bills?
Maybe because his own party's legislators are spending money as though it falls down out of the sky on demand? Whatever else the current crop of GOP Congresspersons and Senators may be, the one thing they are NOT is fiscal conservatives.
1) Gee, having the same party controlling both houses of Congress didn't stop Kennedy, Johnson, and Carter from vetoing bills that weren't sufficently socialist enough for them.
2) Republicans Coolidge and Hoover veteod plenty of bills passed by a Republican controlled Congress. So did Ike during his first term.