There are different qualifications for being a Senator or a President. To be a Senator any type of citizen can qualify. To be a President only a natural born citizen or citizen at the time of the adoption of the Constitution can qualify. That should be obvious.
What is not obvious is that by specifying “citizen” in Article I that the Constitution is describing a fourth type of citizen rather than an inclusive description whereby ANY type of U.S. citizen can qualify. If so then, it should be equally obvious, naturalized citizens and natural born citizens couldn't qualify to be a Senator. That is ludicrous and idiotic.
It is apparent that you and I have different perspectives as to ‘citizens’ as spoken of in the Constitution. For example, I don’t see a ‘citizen’ as mentioned/delineated in the first Article of the Constitution as describing ‘ a fourth type of citizen’- simply incongruous. As to prior to 2008 Obama talk of ‘natural born citizen’ I noted such was not brought forward because there was little concern at the time. There was a report of an incident when Obama was running for Senator from Illinois when a question during a debate of POTUSA eligibility was raised Obama correctly and wisely noted that at the time he was running for Illinois Senator. So even at that time the question of eligibility for POTUSA was up for sidebar discussion.