What if in congress people realized the mistake that passing the bill would do and so refused to go along with it. Other than the effect of the united states population be more against immigration there wouldn’t really be any immediate changes. LBJ continues to pass his progressive policy’s, he pulls us into Vietnam, and Nixon runs as the 1968 presidential nominee and wins. At this point the demographic shifts that happened in our timeline didn’t really effect anything yet, and the thing that made the democrats more willing to push for left-wing policy’s were still hippies so Nixon would humiliate George McGovern and Nixon would continue to do what he did in our timeline. Watergate would still occur, he would resign, and Ford would take his place as the next president. Carter most likely wins and mess up enough that the republicans can put up a dead guy and he’d probably still win. However now this is where they’re could be change from our timeline. Reagan would run and due to his charisma would win again, however one difference that you want to keep in mind is that this is where the hispanic population had grown enough that Reagan tried to bring them under the Republican under the promise of amnesty. This timeline might have dealt with the illegal alien problem by now however if it didn’t then Reagan probably wouldn't have been so quick to give them amnesty since they wouldn’t have had the influence and instead would most likely do the other part of his proposal and start a crackdown on whatever illegals got through and made the border even more difficult to get through. The US at this time would be slightly more conservative than in our time period and now its to be a choice between Bob Dole and George Bush in the primary’s. However this would also be the same as in our timeline, along with Bush’s defeat against Bill Clinton. As things went along the same. The biggest shift would be in the presidential election in 2000, where the nation would be much more social conservative due to policy’s now having to reflect the demographics. This where Buchanan has a chance to win the election since this is where the nation mixup would’ve had more of a say and so Buchanans policy’s would be more popular compared to bush. There’d still be a stronger push for Bush but here there would be more talk about issues that Buchanan would focus on more compared to Bush.