A couple of really stupid beliefs drove this, IMO. Or actually it’s really just one general stupid belief that manifested itself in at least two ways.
First, many Republicans, certainly including Bush, believed that now having both the Congress and the White House, they could keep their own voters and gain Democrat-type voters by funding programs Democrat voters like to extents never before seen. Then these Democrat voters would see that Republicans give them the spending they want, instead of being the mean social-program Scrooges that they always thought Republicans to be, and would switch to voting Republican.
Of course, this was incredibly stupid: Republicans could and did increase spending on given programs by 40% or more, but only to be constantly accused by the in-the-tank-for-the-DNC media of cutting spending on social programs, and just about every Democrat-type voter would remain convinced that the Republicans cut the spending that they want and therefore they still need to vote Democrat.
(Yes, the media and the Democrats really are able to get away with calling large increases “cuts” and they do indeed completely fool most of the public with this.)
Secondly, but same sort of thing, many Republicans including Bush decided that if only they gave amnesty to illegal aliens (though not calling it amnesty) they would win Hispanic voters over to the Republican Party forever.
This was incredibly stupid, not to mention condescending. First, the idea that Hispanic US citizens generally favor laxity for illegal aliens is false, and the idea that those Hispanics that for other reasons choose to vote Democrat would switch to Republican just because amnesty to illegal aliens was granted during a Republican administration was just completely whacked.
And any concept that illegal aliens, on being made legal during a Republican administration, would likely for that reason vote Republican was just beyond stupid.
So these two major planned expansions of their voter base – buying votes with Democrat-style spending, and supposedly winning over the Hispanic vote by granting amnesty – were just total failures.
Which they really should have known they would be.